Optical devices made from radiation curable fluorinated compositions

ABSTRACT

The invention provides organic optical waveguide devices which employ perfluoropolymeric materials having low optical loss and low birefringence. An optical element has a substrate; a patterned, light transmissive perfluoropolymer core composition; and a light reflecting cladding composition on the pattern of the core. Writing of high-efficiency waveguide gratings is also disclosed.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The invention relates to organic optical devices, such as planar singlemode waveguides made from radiation curable materials. Specifically, theinvention relates to low loss, low polarization dependent, devices madefrom fluorohydrocarbon monomers, oligomers, or polymer componentsend-capped with radiation curable ethylenically unsaturated groups, suchas acrylate or methacrylate groups. The devices made from thesematerials show good long term and short term stability, goodflexibility, and reduced stress or crack induced optical scatteringloss.

BACKGROUND OF THE INVENTION

In optical communication systems, messages are transmitted by carrierwaves at optical frequencies that are generated by such sources aslasers and light-emitting diodes. There is interest in such opticalcommunication systems because they offer several advantages overconventional communication systems.

One preferred means for switching or guiding waves of opticalfrequencies from one point to another is by an optical waveguide. Theoperation of an optical waveguide is based on the fact that when alight-transmissive medium is surrounded or otherwise bounded by anothermedium having a lower refractive index, light introduced along the innermedium's axis is highly reflected at the boundary with the surroundingmedium, thus producing a guiding effect.

A wide variety of optical devices can be made which incorporate a lightguiding structure as the light transmissive elements. Illustrative ofsuch devices are planar optical slab waveguides, channel opticalwaveguides, rib waveguides, optical couplers, optical splitters, opticalswitches, optical filters, variable attenuators, micro-optical elementsand the like. These devices are described in more detail in U.S. Pat.Nos. 4,609,252, 4,877,717, 5,136,672, 5,136,682, 5,481,385, 5,462,700,5,396,350, 5,428,468, 5,850,498, and U.S. patent application Ser. No.08/838,344 filed Apr. 8, 1997, the disclosures of which are allincorporated herein by reference.

It is known in the art to make optical waveguides and other opticalinterconnect devices from organic polymeric materials. Whereas singlemode optical devices made from planar glass are relatively unaffected bytemperature, devices made from organic polymers show a far greatervariation with temperature because the refractive index changes muchfaster with temperature in polymeric materials than in glass. Thisproperty can be exploited to make active, thermally tunable orcontrollable devices incorporating light transmissive elements made fromorganic polymers. One type of thermally tunable devices is a directionalcoupler activated by a thermo-optic effect. The thermo-optic effect is achange in the index of refraction of the optical element that is inducedby heat. Thermo-optic effect devices help to provide less costly routerswhen the activation speed of a coupler state is not too high, i.e., whenthe activation speed is in the range of milliseconds.

Unfortunately, most polymeric materials contain carbon-to-hydrogenchemical bonds which absorb strongly at the 1550 nm wavelength that iscommonly used in telecommunication applications. It has long been knownthat fluoropolymers, for example, have significantly reduced absorptionat 1550 nm. While planar waveguides made from fluorinated polyimide anddeuterated polyfluoromethacrylate have achieved single mode losses of aslittle as 0.10 db/cm at 1300 nm, it is relatively difficult to makeoptical devices from these materials. Specifically, thephotolithographic process by which they have been made includes areactive ion etching step. Fluorinated polyimide and deuteratedpolyfluoromethacrylate also have higher losses at 1550 nm, typically onthe order of 0.6 dB/cm.

Photopolymers have been of particular interest for optical interconnectapplications because they can be patterned using standardphotolithographic techniques. As is well known, photolithographyinvolves patternwise exposure of a light-sensitive polymeric layerdeposited on a chosen substrate followed by development of the pattern.Development may be accomplished, for example, by removal of theunexposed portion of the photopolymeric layer by an appropriate solvent.

U.S. Pat. No. 4,609,252 teaches one method of lithographically formingoptical elements using an acrylic photoreactive composition which iscapable of forming a waveguide material upon polymerization. This patentteaches one to utilize polymers with as high a glass transitiontemperature as possible, i.e., 90° C.-220° C., in order to provide forthe greatest operating temperatures. U.S. Pat. No. 5,136,682 teaches theproduction of waveguides using photopolymerizable compositions such asacrylics having a glass transition point, T_(g), of at least 100° C. Theforegoing waveguides, however, suffer from undesirably high optical lossand are not sufficiently flexible.

Among the many known photopolymers, acrylate materials have been widelystudied as waveguide materials because of their optical clarity, lowbirefringence and ready availability of a wide range of monomers.However, the performance of optical devices made from many acrylatematerials has been poor, due to high optical losses, poor resistance toaging and yellowing, and thermal instability of the polymerizedmaterial.

There continues to be a need for low loss radiation curable materialsthat can be used to make optical devices by a more direct process havingfewer manufacturing steps. Specifically, a process is desired that doesnot require a reactive ion etching (RIE) step to develop the pattern ofthe optical element core. Such materials could be used to make opticaldevices by a relatively simple and more direct lithographic procedure.

It is also important that these materials have little or nobirefringence. As is well known in this art, birefringence is thedifference between the refractive index of the transverse electric or TEpolarization (parallel to the substrate surface) and the transversemagnetic or TM polarization (perpendicular to the substrate surface).Such birefringence is undesirable in that it can lead to devices withlarge polarization dependant losses and increased bit error rates intelecommunication systems.

Another tytpe of useful optical device is a waveguide grating.Diffraction gratings, e.g., Bragg gratings, are used in thetelecommunications industry to isolate a narrow band of wavelengths froma broader telecommunications signal. Polymeric planar waveguide gratingshave a number of advantages in terms of their relative ease ofmanufacture and their ability to be tuned over a wide range offrequencies by temperature or induced stress. In addition, such deviceshave the advantage of being easily incorporated into integrated devices.Unfortunately, such gratings in polymeric materials typically are ofrelatively low efficiency. This drawback can result in poor signals withincreased bit error rates. It would, therefore, be beneficial to find amethod of making polymeric planar waveguide gratings with improvedefficiency.

Dense Wavelength Division Multiplexing (DWDM) systems have recentlyattracted a lot of interest because they address the need for increasedbandwidth in telecommunication networks. The use of DWDM systems allowsthe already installed point-to-point networks to greatly multiply theircapacity without the expensive installation of additional optical fiber.DWDM systems can send multiple wavelengths (signals) over the same fiberby using passive optical components to multiplex the signals on the oneend of the line and demultiplex them on the other end of the line.Polymeric materials provide a low-cost, alternative solution to avariety of optical components for DWDM.

WDM devices can be designed by using planar waveguides with gratingsthat can reflect a single wavelength or channel as a building block.These devices can be fabricated with low temperature processes and highthroughput. In this disclosure, we focus on the properties of thisfundamental building block, the fabrication of a grating in a waveguidestructure, outline what we believe is the basic mechanism responsiblefor the grating formation, and its environmental, humidity andtemperature performance.

Prior approaches to meeting these needs have not been completelysatisfactory, and the present invention provides significant andunexpected improvements applicable to this technology in order tosatisfy the materials, process, and device application requirementsnoted above.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided aphotolithographic method of making optical elements comprising:

a) applying a core photopolymerizable composition to a support to form acore photopolymerizable composition layer, said core photopolymerizablecomposition including at least one photoinitiator and at least one corephotopolymerizable monomer, oligomer, or polymer having at least onephotopolymerizable group, said core photopolymerizable monomer,oligomer, or polymer including a perfluorinated substituent;

b) imagewise exposing the core photopolymerizable composition layer tosufficient actinic radiation to effect the at least partialpolymerization of an imaged portion and to form at least one non-imagedportion of said core photopolymerizable composition layer;

c) removing said at least one non-imaged portion without removing saidimaged portion, thereby forming a light transmissive patterned core fromsaid imaged portion;

d) applying an upper cladding polymerizable composition onto thepatterned core; and

e) curing said upper cladding composition, wherein said upper claddingand the core-interfacing surface of said support are each made frommaterials having a lower refractive index than said core.

According to another aspect of the invention, there is provided areactive ion etching method of making optical elements comprising:

a) applying a photopolymerizable composition to a support to form aphotopolymerizable composition layer, said photopolymerizablecomposition including an effective amount of at least one photoinitiatorand at least one photopolymerizable monomer, oligomer, or polymer havingat least one photopolymerizable group, said photopolymerizable monomer,oligomer, or polymer including a perfluorinated substituent;

b) at least partially curing said layer;

c) forming a core by reactive ion etching;

d) applying an upper cladding polymerizable composition onto said core;and

e) at least partially curing said upper cladding composition to form anupper cladding.

According to another aspect of the invention, a light-guiding opticalelement is provided which includes:

a) an organic upper cladding layer;

b) an organic light transmissive core comprising a fluoropolymerincluding at least one perfluorinated substituent;

c) an organic lower cladding layer; and

d) a substrate.

According to another aspect of the invention, a method of transmittingoptical information is provided, the method comprising:

a) providing an information-bearing optical signal; and

b) passing the optical signal through a light-transmissive polymerformed from a perfluorinated radiation curable monomer, oligomer, orpolymer having at least one radiation curable group selected from thegroup consisting of epoxy or ethylenically unsaturated group.

According to another aspect of the invention, a composition is provided,the composition comprising:

a) a first photocurable multifunctional perfluorinated compound having afirst functionality;

b) a second photocurable multifunctional perfluorinated compound havinga second functionality, wherein the difference between said secondfunctionality and said first functionality is at least one; and

c) an effective amount of a photoinitiator.

According to another aspect of the invention, a waveguide grating isprovided, the grating being made from the composition listed above.

Polymerizable compositions for making waveguides in which diffractiongratings can be written are preferably combinations of multifunctionalhalogenated acrylate monomers, oligomers, or polymers. Ideally, thecomonomers are fluorinated species to reduce optical losses through thecured composition. Mixtures of these monomers can form highlycross-linked networks while allowing at the same time the preciseformulation of the refractive index. The ability to control therefractive index to 10⁻⁴ accuracy makes possible the fabrication ofsingle mode waveguide structures with well-defined numerical apertures(NA).

One particular combination of comonomers described in this patentapplication is especially well-suited for writing diffraction gratingsin the waveguides made according to the fabrication methods taught here.Using this material, a single mode channel waveguide has been found tohave a loss of 0.24 dB/cm as determined by the cleave-back method. Thismaterial exhibits low dispersion (on the order of 10⁻⁶ at 1550 nm), lowbirefringenve (≅10−4), and high environmental stability. It also allowsformation of waveguide gratings with excellent filter characteristics.In a 2 cm grating, reflectivity over 99.997% and a 0.2 nm width in thereflection peak at the 3 dB point in reflectivity has been measured.Furthermore, no side lobes have been observed in the reflectionspectrum.

It has also been discovered that a good system-candidate for stronggratings is a mixture of two monomers with different polymerizationrates each of which forms a polymer when fully cured having differentindices of refraction. Comonomers differing in reactive groupfunctionality are also preferred for making gratings in waveguides. Suchsystems perform well when roughly equal weight proportions of eachcomonomer is present in the polymerizable system. More specifically, thepreferred systems includes a photocurable tetra-functional monomer, anapproximately equal weight proportion of a photocurable di-functionalmonomer, and an effective amount of a photoinitiator.

Preferred photopolymerizable monomers, oligomers, and polymers have thestructure

A—R—R_(f)—R′—A

where

R and R′ are divalent or trivalent connecting groups selected from thegroup consisting of alkyl, aromatic, ester, ether, amide, amine, orisocyanate groups;

said polymerizable group, A, is selected from the group consisting of

 CY₂═C(X)COO—,

and

CH₂═CHO—;

 where

Y=H or D, and

X=H, D, F, Cl or CH₃; and

said perfluorinated substitutent, R_(f), is selected from the groupconsisting of

—(CF₂)_(X)—,

—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—, and

—CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)—,

where x is 1-10, m and n designate the number of randomly distributedperfluoroethyleneoxy and perfluoromethyleneoxy backbone repeatingsubunits, respectively, and p designates the number of —CF(CF₃)CF₂O—backbone repeating subunits.

These and other aspects of the invention will become apparent from thedetailed description of the invention set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of a layer of uncured lower claddingpolymerizable composition on a substrate.

FIG. 2 is a section view of the lower cladding polymerizable compositionof FIG. 1 being cured to form the lower cladding layer.

FIG. 3 is a section view of a layer of uncured core polymerizablecomposition on the lower cladding layer of FIG. 2.

FIG. 4 is a section view of the imagewise actinic radiation exposure ofthe core polymerizable composition of FIG. 3.

FIG. 5 is a section view of the core on the lower cladding layer.

FIG. 6 is a section view of a layer of uncured upper claddingpolymerizable composition covering the core and lower cladding.

FIG. 7A is a section view of the imagewise actinic radiation exposure ofthe upper cladding polymerizable composition of FIG. 6.

FIG. 7B is a section view of an optical device resulting fromdevelopment of the upper cladding layer shown in FIG. 7A.

FIG. 8A is a section view of the blanket exposure of the upper claddingpolymerizable composition of FIG. 6 with actinic radiation to form theupper cladding layer.

FIG. 8B is a section view of an optical device resulting from curing ofthe upper cladding layer shown in FIG. 8A.

FIG. 9 is a section view of a layer of uncured core polymerizablecomposition on a substrate.

FIG. 10 is a section view of the imagewise actinic radiation exposure ofthe core polymerizable composition of FIG. 9.

FIG. 11 is a section view of the cured and developed core in contactwith the substrate.

FIG. 12 is a section view of a layer of uncured upper claddingpolymerizable composition covering the core and substrate.

FIG. 13 is a section view of an optical device resulting from imagewiseexposure to actinic radiation and development of the layer of uppercladding polymerizable composition of FIG. 12.

FIG. 14 is a section view of an optical device resulting from blanket ofthe layer of upper cladding polymerizable composition of FIG. 12exposure to actinic radiation.

FIG. 15 is a section view of a layer of uncured lower claddingpolymerizable composition on a substrate.

FIG. 16 is a section view of the lower cladding polymerizablecomposition of FIG. 15 being cured to form the lower cladding layer.

FIG. 17 is a section view of a layer of uncured core polymerizablecomposition on the lower cladding layer of FIG. 16.

FIG. 18 is a section view of the at least partial curing of the corelayer.

FIG. 19 shows the patterned reaction ion etching-resistant layer on theupper cladding layer.

FIG. 20 is a section view of the reaction ion-etching step.

FIG. 21 is a section view of the device after removal of theRIE-resistant layer.

FIG. 22 is a section view of the uniform curing of the upper cladding.

FIG. 23 is a section view of an alternative pattern of the RIE-resistantmaterial suitable for forming a trench.

FIG. 24 is a section view of the reaction ion-etching step forming atrench.

FIG. 25 is a section view showing uncured core polymerizable material inthe trench.

FIG. 26 is a section view of the at least partial curing of the core.

FIG. 27 is a section view of the application of an uncured coating.

FIG. 28 is a section view of the uniform curing of the upper claddinglayer.

FIG. 29 is a section view of a waveguide device having an electrodealigned with the core.

FIG. 30 is a graph showing the dependence of signal level on waveguidelength for an optical waveguide made in accordance with the invention.

FIG. 31 shows absorption spectra for uncured liquid samples ofhexanediol diacrylate and octafluorohexanediol diacrylate.

FIG. 32 shows absorption spectra for uncured liquid octafluorohexanedioldiacrylate and cured octafluorohexanediol diacrylate.

FIG. 33A is a schematic representation of the distribution of monomersbefore grating writing.

FIG. 33B is a graph of the sinusoidal intensity of light passing througha grating writing phase mask.

FIG. 33C-FIG. 33D are schematic representations of monomer diffusion andcreation of a polymer concentration gradient during the writing of agrating in a waveguide.

FIG. 33E is a schematic representation of the polymer concentrationgradient “locked in” after the full cure step of grating writing.

FIG. 33F is a graph of modulation of the refractive index in thewaveguide following writing of the grating.

FIG. 34 shows writing of a grating using a phase mask.

FIG. 35 shows writing of a grating using a two-beam interference set-up.

FIG. 36 is a photo-differential scanning calorimetry plot of extent ofpolymerization versus time for two comonomers that can be used in theinvention.

FIG. 37 is a plot of transmitted power versus wavelength near 1550 nmfor a reflection waveguide grating made in accordance with theinvention.

FIG. 38 is a plot demonstrating the strong linear dependence of thereflected wavelength of a grating made in accordance with the inventionwith temperature.

FIG. 39 is a plot of the dependence of the change in the Braggwavelength of a grating made in accordance with the invention withtemperature (dλ_(B)/dt) on the coefficient of thermal expansion of thewaveguide substrate.

FIG. 40 is the flowsheet for an algorithm useful in screening comonomersystem candidates for use as a grating material.

FIG. 41 is a plot generated by a computer program implementing theflowsheet of FIG. 40 which shows the fraction of a monomer formed into apolymer for four comonomer system candidates under evaluation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention will now be described in more detail by way of examplewith reference to the embodiments shown in the accompanying figures. Itshould be kept in mind that the following described embodiments are onlypresented by way of example and should not be construed as limiting theinventive concept to any particular physical configuration.

According to a preferred embodiment of the invention, a film of a lowercladding polymerizable composition 1 is applied to the surface of asubstrate 4, as shown in FIG. 1. The film may be applied in a number ofdifferent ways known in the art, such as spin coating, dip coating, slotcoating, roller coating, doctor blading, liquid casting or the like.Generally, the lower cladding polymerizable composition is applied at athickness of from at least about 0.01 microns, preferably at least about1 micron, to about 10 microns or more.

While the lower cladding can be made from any material having arefractive index lower than the core, the most preferred lower claddingmaterial is a fluoropolymeric composition as described below. A low losscladding material, such as a fluorinated polymer, is preferred in partbecause while the majority of the optical signal is transmitted throughthe core, a portion of the signal is transmitted through the claddingmaterial.

Preferably, the lower cladding polymerizable composition is curable byheat and/or actinic radiation. More preferably, the lower claddingpolymerizable composition is photocurable by actinic radiation. Uponexposure to an appropriate source of radiation 5 effective to at leastpartially cure the lower cladding polymerizable composition, as shown inFIG. 2, a lower cladding 6 is formed on the substrate 4. Preferably, theradiation 5 is a blanket or overall, non-imagewise exposure ofultraviolet radiation.

To form the light transmissive region or core, a thick or thin film of acore polymerizable composition 2 is applied to the lower cladding 6, asshown in FIG. 3. Generally, the core polymerizable composition isapplied at a thickness of from about 1 micron to about 1 mm, preferablyfrom about 5 microns to about 500 microns. Preferably, the corepolymerizable composition is photopolymerizable, i.e., curable byexposure to actinic radiation. As described more fully below, thepreferred core polymerizable compositions is a low loss fluorinatedmaterial.

In one embodiment of the invention, the core polymerizable compositionlayer is imagewise exposed to a suitable form of curing radiation 5 thatis effective to at least partially cure the exposed, image portion ofthe core polymerizable composition layer without substantially curingthe unexposed, non-image areas of the core polymerizable compositionlayer, as shown in FIG. 4. Preferably, the curing radiation 5 is actinicradiation, more preferably ultraviolet radiation, exposed through a corephotomask 7. The position and dimensions of the light transmissive coreis determined by the pattern of the actinic radiation upon the surfaceof the film. The radiation pattern preferably is chosen so that thepolymerizable composition is polymerized in the desired pattern and sothat other regions of the core polymerizable film remain substantiallyunreacted. If, as in a preferred embodiment, the polymerizablecomposition is photocurable, the photopolymer is conventionally preparedby exposing the core polymerizable composition to actinic radiation ofthe required wavelength and intensity for the required duration toeffect the at least partial curing of the photopolymer.

In one preferred embodiment, the core polymerizable composition is notfully cured, but is only partially polymerized prior to applying theupper cladding polymerizable composition. Partial polymerization of thecore polymerizable composition layer prior to application of the uppercladding polymerizable composition layer allows the two compositions tointermingle at their interface. This improves adhesion of the two layersand also reduces optical loss by reducing scattering at the interface ofthe core and cladding. Additionally, by not fully polymerizing the coreat this point in the process allows for the writing of diffractiongratings in the core layer in a subsequent step, if desired, asdescribed more fully below. The same partial polymerization techniquecan be used at the lower cladding/core interface as well by not fullycuring the lower cladding polymerizable composition layer beforeapplying the core polymerization composition layer.

After the core polymerizable composition has been at least partiallypolymerized to form the predetermined pattern of the polymer on thesurface of the lower cladding, the pattern is developed by removing thenonimage areas and leaving intact the predetermined pattern of core 8,as shown in FIG. 5. Any conventional development method can be used, forexample, flushing with a solvent for the unirradiated composition. Suchsolvents include polar solvents, such as alcohols and ketones. The mostpreferred solvents are acetone, methanol, propanol, tetrahydrofuran andethyl acetate. For highly fluorinated materials, the preferred solventis Galden® HT-110, a perfluorinated ether available from Ausimont USA.

Although FIG. 4-FIG. 5 show the formation of just one core using aphotomask having one transparent image-forming region, the skilledartisan will appreciate that multiple spaced-apart cores could be formedon the lower cladding simultaneously using a photomask having multipletransparent image-forming regions or similar devices capable of causingthe exposure of multiple image areas.

Two alternative methods of forming the upper cladding will now bedescribed. In each case, a film of upper cladding polymerizablecomposition 3 is applied over the lower cladding 6 and core 8, as shownin FIG. 6. Like the lower cladding layer, while the upper cladding canbe made from any material having a refractive index lower than the core,the most preferred upper cladding material is a fluoropolymericcomposition as described below. As noted above, a low loss claddingmaterial is preferred in part because a portion of the optical signal istransmitted through the cladding material.

Preferably, the upper cladding polymerizable composition is curable byheat and/or actinic radiation. More preferably, the upper claddingpolymerizable composition is photocurable by actinic radiation. Thepreferred form of actinic radiation is ultraviolet radiation.

The upper cladding polymerizable composition layer is at least partiallycured by an appropriate form of curing radiation 5. In one method shownin FIG. 7A-FIG. 7B, actinic radiation is exposed through an imagingcladding photomask 11 to form an imaged, at least partially cured regionand unexposed, uncured regions. The upper cladding 9 is developed byremoval of the unexposed, uncured regions by an appropriate solvent, forexample. The resulting core 8 and upper cladding 9 form a ridge-likestructure extending above the plane of the lower cladding 6 andsubstrate 4. Upper cladding 9 covers the top and sides of the core 8.This type of upper cladding 9 is advantageous since its core 8 exhibitslow internal stresses. Preferably, the core 8 is entirely enveloped bythe lower cladding 6 and upper cladding 9. The upper and lower claddingsmay, of course, be referred to collectively as simply the cladding.

In an alternative method shown in FIG. 8A-FIG. 8B, the upper claddingpolymerizable composition layer 3 is simply blanket, overall, ornon-imagewise exposed to a suitable form of curing radiation 5 effectiveto at least partially cure the upper cladding polymerizable composition,as shown in FIG. 8A, to form a planar upper cladding layer 10, as shownin FIG. 8B. Preferably, the core 8 is entirely enveloped by the lowercladding 6 and upper cladding 10.

So that the resulting structure functions as a waveguide by guidinglight through the core, the polymerizable compositions are selected sothat the refractive index of the lower cladding (fully cured) and therefractive index of the upper cladding (fully cured) are both less thanthe refractive index of the core (fully cured). The refractive indicesof the lower and upper cladding layers can be the same or different.Preferably, the lower cladding has a similar T_(g) property as that ofthe upper cladding, but it need not be made from the identicalcomposition. The lower cladding polymerizable composition and processingconditions are selected such that the T_(g) of the polymerized lowercladding layer preferably ranges from about 60° C. or less, morepreferably about 40° C. or less and even more preferably about 25° C. orless. Preferably, the refractive index of the upper cladding will be thesame as that of the lower cladding. The lower cladding polymerizablecomposition and the upper cladding polymerizable composition may be thesame material.

If diffraction gratings are not to be written in the waveguide, afterapplication of the upper cladding polymerizable composition, anyunpolymerized or not fully polymerized portions of the upper cladding,lower cladding or core layers may be subjected to a hard curing by ablanket or overall exposure to actinic radiation such that they aresubstantially fully polymerized. In this manner, the core and claddingcompositions intermix at their interface and can be mixed in any desiredproportions to fine tune the refractive indices of the cladding, coreand the overall device and insure good adhesion between the layers bycovalent bonding.

If diffraction gratings are to be written in the partially curedwaveguide, reasonable measures should be taken to protect the waveguidelaminate from further polymerization, such as that induced by actinicradiation or heat, until the grating writing step.

In some cases, for example, when the refractive index of the substrateis less than that of the core, a lower cladding will not be necessary.One process of making a light-guiding optical device without a lowercladding is illustrated in FIG. 9-FIG. 14. To form the core 8, a film ofa core polymerizable composition 2 is applied to the substrate 4, asshown in FIG. 9. The core polymerizable composition layer 2 is imagewiseexposed, e.g., through core photomask 7, to a suitable form of curingradiation 5, e.g., ultraviolet radiation, that is effective to at leastpartially cure the exposed, image portion of the core polymerizablecomposition layer without substantially curing the unexposed, non-imageareas of the core polymerizable composition, as shown in FIG. 10. Upondevelopment of the imaged area by removal of the uncured non-image area,as by an appropriate solvent for the uncured non-imaged area but not forthe cured image area, a core 8 is formed on the substrate 4 without anintervening lower cladding layer between the core and substrate, asshown in FIG. 11.

The upper cladding layers 9, 10 can be formed in accordance with thedescription above. That is, an upper cladding polymerizable composition3 is applied over the substrate 4 and core 8, as shown in FIG. 12. Theupper cladding polymerizable composition layer 3 may then be cured by anappropriate form of curing radiation to form an at least partially curedupper cladding layer. In one variation of this method similar to thatshown in FIG. 7A, an upper cladding photomask, an appropriately selectedcuring radiation effective to at least partially cure the upper claddingpolymerization composition, and development of the imaged area can beused to form the upper cladding layer 9 to produce the lowercladding-free ridge-like optical device 13 shown in FIG. 13.Alternatively, the upper cladding polymerizable composition layer issimply blanket-, overall-, or non-imagewise-exposed to a suitable formof curing radiation, such as ultraviolet radiation, by a method similarto that shown in FIG. 8A, to form planar upper cladding 10, as shown inFIG. 14.

In addition to using these materials for making planar waveguides by thelithographic method described above, reactive ion etching (RIE) may alsobe used to make planar waveguides in a manner similar to that describedin the Journal of Lightwave Technology, Vol. 16, June 1998, page 1024.

A representative procedure for making waveguides by a RIE method isshown in FIG. 15-22. A uniform polymerized core layer 12 is provided ontop of a polymerized lower cladding layer 6 atop substrate 4 usingactinic radiation 5 as described previously and as shown in FIG. 15-FIG.18. Preferably, the lower cladding and/or core layers are partiallyrather than fully polymerized to improve interlayer adhesion, and toallow for subsequent writing of a grating in the waveguide, if desired.A patterned RIE resistant layer (mask) 13 could then be applied on topof the core layer 12 by procedures known in the art, such asconventional photolithographic or other type patterning methods, asshown in FIG. 19. The patterning preferably would be selected such thatthe RIE resistant layer 13 would lie above the area where the waveguidecore is desired. Such an RIE resistant layer could be composed of aphotoresist, a dielectric layer, or a metal as is familiar to thoseskilled in the art. Reactive ion etching would then be employed usingion beams 14 to remove the core material down to the level of the lowercladding, as shown in FIG. 20. The area of the core protected from theion beams by the RIE resistant layer would remain after removal of theRIE resistant layer by conventional techniques, as indicated by core 8at FIG. 21, thereby producing a raised rib structure of waveguide core 8made of the core material. A top coat of upper cladding material couldbe applied and cured using actinic radiation 5 to form upper claddinglayer 10 to complete the waveguide, as shown in FIG. 22.

As mentioned previously, partial polymerization of the layers could beused to improve the interlayer adhesion, reduce optical losses, andallow for writing of a grating in the waveguide in a subsequent step. Itis especially advantageous to leave the lower cladding layer only partlypolymerized before the core layer is applied. In this case thesubsequent radiation dose applied to the core, as shown in FIG. 18, alsoacts to further polymerize the lower cladding and strengthens the bondbetween the layers.

Another method of making waveguides by RIE also begins by at leastpartially polymerizing a lower cladding coating layer 1 applied to asubstrate 4 with actinic radiation 5 to form a lower cladding layer 6,as previously described and shown in FIG. 15 and FIG. 16. An RIEresistant layer 13 could then be patterned on top of the lower claddinglayer 6, as shown in FIG. 23. The lower cladding layer 6 in FIG. 23 isrelatively thicker than the lower cladding layer 6 shown in FIG. 16 forclarity in describing the method involving a RIE step. The figures arenot drawn to scale.

The resistant layer 13 is preferably applied in vertical registrationwith the portions of the lower cladding layer that will remain afterformation of the waveguide core. Reactive ion etching could then beperformed using ion beams 14 to remove the unprotected portions of lowercladding layer 6 down to a desired depth, i.e., to remove the lowercladding layer except where the RIE resistant layer was patterned, toproduce a trench 15, as shown in FIG. 24. In cases where the index ofrefraction of the substrate is higher than that of the cured corematerial, a residual portion 16 of the lower cladding is not removedduring the ion etching step. In cases where the substrate has a lowerrefractive index than the cured core, the lower cladding layer may beremoved down to the level of the substrate, if desired (not shown). Thetrench 15 could then be at least partially filled with core material 1,as shown in FIG. 25. The uncured core material could then be at leastpartially cured by actinic radiation 5 to form a waveguide core 8, asshown in FIG. 26. Subsequently, an upper cladding coating layer 2 can beapplied by methods previously described, for example, as shown at FIG.27. As described previously, by only partially polymerizing the layers,the interlayer adhesion and the optical losses can be improved, andgratings can later be written in the waveguide, if desired. The uppercladding coating layer 2 may then be uniformly cured by actinicradiation to form an upper cladding 12, as shown in FIG. 28.

Further techniques that may be used include micro replication asexemplified in U.S. Pat. No. 5,343,544, the disclosure of which isincorporated herein by reference, direct laser writing similar to thatdescribed in the Journal of Lightwave Technology, Vol. 14, No. 7, July1996, page 1704, and laser ablation similar to that described in U.S.Pat. No. 5,106,211, the disclosure of which is incorporated herein byreference.

Insofar as the combined lower cladding/substrate of FIG. 5 or thesubstrate of FIG. 11 each serves to support the core, either structuremay be referred to as a core support.

Regardless of the specific manner of making the waveguide device, i.e.,with or without a RIE step, optional additional layers may also beemployed above or below the upper cladding or lower cladding,respectively. For example, one or more conductive layers, such aselectrode 17 shown in FIG. 29, could be applied above the upper claddinglayer for use in thermo-optic applications using patterning or othermethod known to those skilled in the art. Preferably, the electrode 17is aligned in registration with the core. The conductive layer may bemade of metal or a conductive polymer, for example.

If the core has a refractive index that is lower than the substratematerial, it is necessary to first form a layer of material having arefractive index lower than the refractive index of the core. Such alayer may be referred to as a buffer layer and may be comprised of, forexample, a semiconductor oxide, a lower refractive index polymer (as inthe method shown by FIG. 1-FIG. 6), or a spin-on silicon dioxide glassmaterial.

The substrate may be any material on which it is desired to establish awaveguide. The substrate material may, for example, be selected fromglass, quartz, plastics, ceramics, crystalline materials andsemiconductor materials, such as silicon, silicon oxide, galliumarsenide, and silicon nitride. Formation of the optical elements onwafers made of silicon or other compositions are specificallycontemplated. Silicon wafers are preferred substrates in part due totheir high surface quality and excellent heat sink properties. Toimprove adhesion of the photopolymer to the silicon wafer, the wafer maybe cleaned and treated with silane or other adhesion promoter, ifdesired. The substrate may or may not contain other devices, eithertopographical features such as grooves or electrical circuits, orelectro-optic devices such as laser diodes.

A preferred plastic substrate is a urethane-coated polycarbonatesubstrate which is described in provisional patent application Ser. No.60/121,259 filed on Feb. 23, 1999, for “Control of Temperature DependentPlanar Polymeric Waveguide Devices through the use of Substrate andSuprastrate Layers with Specific Coefficients of Thermal Expansion,” thedisclosure of which is incorporated herein by reference.

The terms “lower cladding” and “upper cladding” refer to cladding layerspositioned on opposite sides of a core. Accordingly, the terms “lowercladding” and “upper cladding” are used here without regard to theirposition relative to any gravitational field.

The terms “lower cladding polymerizable composition,” “upper claddingpolymerizable composition,” and “core polymerizable composition”correspond to the third, second, and first compositions, respectively,of co-pending patent application Ser. No. 08/838,344 filed Apr. 8, 1997.Compositions suitable for use as a lower cladding, upper cladding, orcore polymerizable composition are not limited, however, to thecompositions described in the Ser. No. 08/838,344 application.

The polymerizable compositions suitable for use in this inventioninclude a polymerizable compound or mixture of two or more polymerizablecompounds and other additives, such as photoinitiators. Thepolymerizable compounds which can be used to form the cladding and coremay be monomers, oligomers, or polymers which are additionpolymerizable, nongaseous (boiling temperature above 30° C. atatmospheric pressure) compounds containing at least one and preferablytwo, three, four, or more polymerizable groups, e.g., an epoxy orethylenically unsaturated group, and are capable of forming highmolecular weight polymers by radical cation initiated or free radicalinitiated, chain propagating addition polymerization. Such compounds arewell known in the art. The polymerizable compounds may be polymerized bythe action of actinic radiation, heat, or both. The polymerizablecompounds that can be polymerized by the action of actinic radiation maybe referred to as being photopolymerizable, photocuring, photocurable,radiation curable, or the like. In one preferred embodiment, at leastone of the polymerizable compounds contains at least two polymerizablegroups per polymerizable monomer, oligomer, or polymer, e.g., at leasttwo epoxy or ethylenically unsaturated groups. Accordingly, thepreferred polymerizable compounds are multi-functional, i.e.,di-functional, tri-functional, tetra-functional, etc., in that theyinclude at least two polymerizable functional groups. At least one ofthe polymerizable compounds may contain, for example, four polymerizablegroups, in particular, four epoxy or four ethylenically unsaturatedgroups. The polymerizable compounds preferably are selected so thatafter exposure, they yield the below described T_(g) and refractiveindex.

A preferred polymerizable composition includes at least onemulti-functional polymerizable compound and at least one otherhigher-order multi-functional polymerizable compound. For example, onepolymerizable compound in the polymerizable composition may be adi-functional polymerizable compound while another polymerizablecompound in the composition may be a tri-functional, tetra-functional,penta-functional, or higher functionality polymerizable compound.Preferably, the difference in functionality between at least one of thepolymerizable compounds and at least one other polymerizable compound inthe polymerizable composition is at least two, e.g., a di-functionalcompound and a tetra-functional compound, a tri-functional compound anda penta-functional compound, etc., or a mono-functional compound and atri-functional or higher functionality compound.

In order to form cross-linked polymers, at least one polymerizablecompound in the polymerizable composition must be at leastdi-functional. Monofunctional halogenated or non-halogenated monomerscan also be used, but there may be some long-term outgassing or materialmigration of any non-reacted monomers of this type. By using monomersthat are at least di-functional, the likelihood of a monomer not havingat least partially reacted is dramatically reduced.

In polymerizable compositions including more than one polymerizablecompound, the compounds are preferably present in roughly equal weightproportions. For example, in a two polymerizable-compound composition,the composition preferably includes from about 40 to about 60 wt. % ofone compound and from about 40 to about 60 wt. % of the other compound,based on the total weight of the polymerizable compounds in thecomposition. More preferably, the composition includes from about 45 toabout 55 wt. % of one compound and from about 45 to about 55 wt. % ofthe other compound, based on the total weight of the polymerizablecompounds in the composition. Most preferably, the composition includesabout 50 wt. % of each of the two polymerizable compounds based on thetotal weight of the polymerizable compounds. Similarly, in a threepolymerizable-compound composition, the composition preferably includesfrom about 25 to about 40 wt. % of each of the three compounds based onthe total weight of the polymerizable compounds in the composition. Morepreferably, the composition includes about 33 wt. % of each of the threepolymerizable compounds based on the total weight of the polymerizablecompounds in the polymerizable composition. Four or more polymerizablecompounds may be formulated in a polymerizable composition, if desired.

An especially preferred polymerizable composition for making waveguidelaminates is one including roughly equal weight proportions of two ormore multi-functional polymerizable compounds at least two of whichcompounds differ in functionality by at least two. Such a polymerizablecomposition would preferably include an effective amount of one or morepolymerization initiators. More preferably, the multi-functionalpolymerizable compounds differing in functionality would bephotopolymerizable in the presence of an effective amount of one or morephotoinitiators and an effective dosage of actinic radiation, such asultraviolet light. Furthermore, the multi-functional polymerizablecompounds in the composition would preferably polymerize at differentrates.

The photopolymerizable compositions may be used to make partially curedwaveguide laminates according to the methods described above.Diffraction gratings, e.g., Bragg diffraction gratings, can then bewritten in these partially cured waveguide laminates using a lightsource, such as a laser, and a phase mask or two-beam interferenceset-up. One such composition suitable for use in making Braggdiffraction gratings in planar polymeric waveguides is described atExample G below. Methods for writing gratings in the waveguide laminateswill be disclosed in greater detail after describing the polymerizablecompositions.

Photopolymerizable compounds are preferred for use in the polymerizablecompositions. In particular, multifunctional acrylate monomers arepreferred. The generalized structure of the multifunctional acrylates isgiven by structure (I):

For the core, m preferably ranges from 1 to about 6; R₂ is H or CH₃, andR₁ may be a linkage of aliphatic, aromatic or aliphatic and aromaticmixed organic molecular segments. Preferably R₁ is an alkylene, alkyleneoxide, arylene oxide, aliphatic polyether or polyester moiety and R₂ isH. To ensure solvent resistance of the cured film and high contrastphotolithography, crosslinked polymers are preferred, so multifunctionalacrylate monomers (m≧2) are preferred.

One of the embodiments of this invention decreases stress inducedscattering optical loss of the final waveguiding device by usingflexible, low glass transition temperature (T_(g)) polymers. It is knownin the art that the glass transition temperature (T_(g)) of acrosslinked polymer depends on the crosslinking density and thestructure of the linkage between crosslinking points. It is also knownthat both low crosslinking density and flexible linkage require a lowT_(g). To ensure low crosslinking density, monomers with 1≦m≦3,preferably m=2, and long linkage segments between two ethylenicallyunsaturated functionalities are preferred. For this invention, longlinkage segments are those which have an average molecular chain lengthof at least about 4 carbon atoms or larger and preferably 6 or larger.Suitable flexible linkage structures include alkylenes with chain lengthlarger than about 3 carbon atoms, poly(ethylene oxide), poly(propyleneoxide), ethoxylated bisphenol A, polyethers, thioethers, aliphatic andaromatic hydrocarbons, ethers, esters and polysiloxanes, etc. These mayoptionally be substituted with any pendant group which does notsubstantially detract from the ability of the polymerizable compound tophotopolymerize. Suitable substituents nonexclusively include alkyl,aryl, alkoxy and sulfoxide groups, etc. To ensure high resistance tothermal degradation and discoloration, thermally stable molecularstructures of R₁ are preferred. Such R₁ segments are preferablysubstantially free of thermally susceptible moieties such as aromaticurethane and amide groups. To ensure low birefringence, R₁ linkages withlow stress optic coefficient and optical polarizability are preferred.

For the cladding, the acrylate is also as described above, however, theaverage molecular chain length between ethylenically unsaturatedfunctionalities is preferably about 6 carbon atoms or longer, preferably8 or longer and more preferably 12 or longer. Suitable flexible linkagestructures include alkylenes with chain length larger than 6 carbonatoms, poly(ethyleneoxide), poly(propylene oxide) and ethoxylatedbisphenol A.

Preferred polymerizable components for both the cladding and the coreare esters and partial esters of acrylic acid and of aromatic andaliphatic polyols containing preferably 2 to 30 carbon atoms. Thepartial esters and esters of polyoxyalkylene glycols are also suitable.Examples are ethylene glycol diacrylate, diethylene glycol diacrylate,triethylene glycol diacrylate, tetraethylene glycol diacrylate,polyethylene glycol diacrylates and polypropylene glycol diacrylateshaving an average molecular weight in the range from 200 to 2000,propylene glycol diacrylate, dipropylene glycol diacrylate, (C₂ to C₄₀)alkane diol diacrylates such as hexanediol diacrylate, and butanedioldiacrylate, tripropylene glycol diacrylate, trimethylolpropanetriacrylates, ethoxylated trimethylolpropane triacrylates having anaverage molecular weight in the range from 500 to 1500, pentaerythritoldiacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate,dipentaerythritol diacrylate, dipentaerythritol triacrylate,dipentaerythritol tetraacrylate, dipentaerythritol pentaacrylate,dipentaerythritol hexaacrylate, tripentaerythritol octaacrylate,sorbitol triacrylate, sorbitol tetraacrylate, sorbitol pentaacrylate,sorbitol hexaacrylate, oligoester acrylates, glycerol di- andtriacrylate, 1,4-cyclohexane diacrylate, bisacrylates of polyethyleneglycols having an average molecular weight from 100 to 1500, andmixtures of the above compounds. Preferred multifunctional acrylateoligomers include, but are not limited to acrylated epoxies, acrylatedpolyurethanes and acrylated polyesters. Preferred photopolymerizablecompounds are aryl acrylates. Illustrative of such aryl acrylatemonomers are aryl diacrylates, triacrylates and tetraacrylates as, forexample, di, tri and tetraacrylates based on benzene, naphthalene,bisphenol-A, biphenylene, methane biphenylene, trifluoromethanebiphenylene, phenoxyphenylene, and the like. The preferred aryl acrylatemonomers are multifunctional aryl acrylates and more preferred arylacrylate monomers are di, tri and tetra acrylates based on thebisphenol-A structure. Most preferred aryl acrylate monomers arealkoxylated bisphenol-A diacrylates such as ethoxylated bisphenol-Adi-acrylate, propoxylated bisphenol A diacrylates and ethoxylatedhexafluorobisphenol-A diacrylates. The aryl acrylate monomers of choiceare ethoxylated bisphenol-A diacrylates. Preferred polymerizablecomponents are monomers having the structure (II):

In a preferred embodiment, for the core, n is about 10 or less,preferably about 4 or less and most preferably about 2 or less. In onepreferred embodiment, for the cladding, n is about 2 or more, preferablyabout 4 or more and most preferably about 10 or more. Also useful areacrylate-containing copolymers which are well known in the art. In onepreferred embodiment, the cladding layer comprises a polymerizablecomponent which has the ethoxylated bisphenol-A diacrylate structure(II) shown above wherein 1≦n≦20, preferably 4≦n≦15, and more preferably8≦n≦12. In the most preferred embodiment of the invention, the secondphotosensitive composition is miscible with the polymerized firstphotosensitive composition at their interface.

Preferred polymerizable components for making low loss waveguides aremultifunctional monomers having the structure (III):

A—R—R_(f)—R′—A  (III)

where

R and R′ are divalent or trivalent connecting groups selected from thegroup consisting of alkyl, aromatic, ester, ether, amide, amine, orisocyanate groups;

A is a polymerizable group, such as

CY₂═C(X)COO—

 or

 CH₂═CHO—

 or

 or

 where

Y=H or D, and

X=H, D, F, Cl or CH₃; and

R_(f) is a perfluorinated substitutent, such as

—(CF₂)_(x)—,

where x is 1-10,

—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—,

 or

—CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)—,

where m and n designate the number of randomly distributedperfluoroethyleneoxy and perfluoromethyleneoxy backbone repeatingsubunits, respectively, and p designates the number of —CF(CF₃)CF₂O—backbone repeating subunits, where m, n, and p are integers 0, 1, 2, 3,. . . Preferably, x is 4-6.

Accordingly, the polymerizable compounds suitable for use in theinvention include, for example, polydifluoromethylene diacrylates,perfluoropolyether diacrylates, perfluoropolyether tetraacrylates, andchloroflurodiacrylates. One suitable chlorofluoroduacrylate is thecompound

CH₂═CHCO₂CH₂CF₂(CFCICF₂)_(n)CH₂O₂CCH═CH₂.

One purpose in incorporating chlorine atoms in the structure is to raisethe refractive index to that of a fully fluorinated compound withoutincreasing the optical loss values.

In addition to the groups listed above, the polymerizable group A mayalso be a thiol group. Thiol-polyene UV curable systems can also beused. Without intending to be bound to any particular explanation forthis curing system, the mechanism for the thiol-polyene reaction isgenerally understood as follows:

PI•+RSH→PI−H+RS•RS•+H₂C═CHR′→RSCH₂−{dot over (C)}HR′RSCH₂−{dot over(C)}HR′+RSH→RSCH₂−CH₂R′+RS•

In the first step of this reaction, a photoinitiator-generated freeradical removes a proton from a thiol group to create a thiol radical.This thiol radical then reacts with a carbon double bond to create aradical intermediate. The radical intermediate then abstracts a protonfrom another thiol forming a thiol ether and another thiol radical. Inthis reaction, one thiol reacts with one carbon double bond. Also, for apolymer to develop, both the thiol and the alkene must be at leastdi-functional. In order to get a cross-linked polymer, it is necessarythat at least one of the components be at least tri-functional.

The polymers generated by this reaction generally have good physicalproperties. Their shrinkage is also likely to be low. Unlike acrylates,this reaction is fairly insensitive to oxygen, but does have terminationsteps that occur when two radicals come together. These propertiessuggest that these materials may be able to produce reasonablelithographic resolution. The main problem with this approach is theavailability of low-loss starting materials. Since these materialspreferably formulated on a 1:1 thiol:alkene basis, varying refractiveindex requires at least three different compounds instead of two asexemplified elsewhere in this application.

When the perfluorinated substitutent group R_(f) is

—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—,

the ratio m/n preferably varies from about 0.5 to about 1.4. A sample ofthese materials will include a distribution of molecules havingdifferent numbers of repeating subunits. In such a sample, the averagevalue of m preferably falls within the range of from about 6.45 to about18.34, and the sample average value of n preferably falls within therange of from about 5.94 to about 13.93. Most preferably, the ratio m/nis about 1 and the sample average values of m and n are each about 10.3.

Preferably, the connecting group R is —CH₂— or —CH₂C(A)HCH₂OCH₂— and theconnecting group R′ is —CH₂— or —CH₂OCH₂C(A)HCH₂—, where A is defined asabove. In light of this disclosure, the skilled artisan will recognizethat a wide variety of connecting groups R and R′ could be used inaddition to those listed here.

A particularly preferred polymerizable compound for use in the inventionhas the structure

Preferably, the ratio m/n is about 1 and the molecular weight is betweenabout 2000 and 2800.

When selecting the polymerizable compounds to be used in each of thecore and the cladding, it is important that the core which results afterfull polymerization has a higher refractive index than that of thecladding after polymerization. Preferably the core has a refractiveindex in the range of from about 1.3 to about 1.6, or more preferablyfrom about 1.35 to about 1.56. Preferably the cladding has a refractiveindex in the range of from about 1.29 to about 1.58, or more preferablyfrom about 1.34 to about 1.55. Although the cladding and core may becomprised of structurally similar compositions, it is clear that inorder for the cladding to have a refractive index which is lower thanthe refractive index of the core, they must have different chemicalcompositions for any individual application. In addition, as notedabove, if the chosen substrate has a refractive index which is greaterthan that of the core, then a buffer layer is required and the buffermust have a refractive index which is lower than that of the core.

In selecting other monomers and oligomers that may be suitable forforming planar light guiding devices, the following observations shouldbe considered. For high purity fluorinated acrylates, the majority ofthe absorbance at 1550 nm is a result of carbon-to-hydrogen bonds. Theabsorption spectra for the non-fluorinated compound hexanedioldiacrylate (HDDA) and the fluorinated compound octafluorohexanedioldiacrylate (OFHDDA), in which eight hydrogen atoms are replaced byfluorine, as shown in FIG. 31, illustrate this point. The small peaksaround the 1550 nm and 1310 nm regions of the spectra are characteristicof uncured liquids. After cure, virtually all of these fluctuations areeliminated, as shown in the spectrum of cured octafluorohexanedioldiacrylate appearing at FIG. 32. Most of the elimination is probably dueto the conversion of the carbon double bonds to carbon single bonds asthe acrylate cures. Further, differences in the baseline absorbancevalues are believed to be the result of the higher level of scatteringin the solid sample. Such scattering is an artifact of the way in whichthe sample was made and the thickness variation in the sample. Actualwaveguide losses for this material would be substantially lower thanindicated in FIG. 32.

In evaluating the relative merits of a particular acrylate based on itsstructure, it is useful to determine the molar concentration of hydrogenbonds for a particular candidate material. Since the absorption loss (indB/cm) is determined by the relation${{{Absorption}\quad {loss}} = {\frac{10 \cdot A}{b} = {{10 \cdot ɛ}\quad c}}},$

where A is the absorbance, ∈ is the molar absorptivity, b is the pathlength in centimeters, and c is the molar concentration, the lower themolar concentration, the lower the absorption loss. Since almost all ofthe loss comes from carbon-to-hydrogen bonds, the molar concentration ofhydrogen (C_(H)) for a particular monomer can be calculated using thenumber of hydrogens per molecule (H), the molecular weight of themonomer (Mw), and its density (ρ), as shown by the equation:$C_{H} = \frac{H \cdot \rho \cdot 1000}{Mw}$

While an exact relationship between C_(H) and the loss measurement in aparticular waveguide is unlikely, this relation gives a first indicationof which materials may be useful in lowering loss values. When makingthese calculations, it is most appropriate to use the sensitivity of acured film of the monomer since it is the loss of the cured film that isof greatest interest. However, since the measure of density of suchfilms is difficult, the density of the liquid could be used with theunderstanding that it does introduce some error.

Preferably, the photopolymerizable compounds to be used in the waveguidecore produce a core which after polymerization has a glass transitiontemperature of about 80° C. or less and more preferably about 50° C. orless. Furthermore, it is preferred that the polymerizable compounds tobe used in the waveguide cladding produce a cladding which afterpolymerization has a glass transition temperature of about 60° C. orless, more preferably about 40° C. or less and most preferably about 25°C. or less. Preferably, the polymerizable compounds included in thecladding polymerizable compositions are also photopolymerizable. Theparticular T_(g) may be easily obtained by the skilled artisan bycharacterization and selection of the polymerizable component. Thisdepends on such factors as the molecular weight, number of sites ofunsaturation, and crosslink density of the polymerizable component. Asingle polymerized component may itself have the desired T_(g), or thepolymerizable component may be tailored by blending mixtures ofpolymerizable monomer, oligomers and/or polymers having the desiredT_(g). The T_(g) may also be controlled by varying the irradiationexposure time and temperatures at which polymerization is conducted.

The polymerizable compound is present in each polymerizable compositionin an amount sufficient to polymerize upon exposure to sufficient heatand/or actinic radiation. The amount of the photopolymerizable compoundin the composition may vary widely and amounts normally used inphotopolymerizable compositions for use in the preparation ofphotopolymers for use as the light transmissive element of lighttransmissive devices may be used. The amount of photopolymerizablecompound is generally used in an amount of from about 35 to about 99.9%by weight of the composition. In the preferred embodiment, one or morephotopolymerizable compounds in the overall photopolymerizablecomposition account for from about 80% to about 99.5% by weight, mostpreferably from about 95 to about 99.5% based on the weight of theoverall composition.

Each light sensitive composition further comprises at least onephotoinitiator. The photoinitiator can be a free radical generatingaddition polymerization initiator activated by actinic light and ispreferably thermally inactive near room temperature, e.g., from about20° C. to about 80° C. Any photoinitiator which is known tophotopolymerize acrylates can be used. Preferred photoinitiatorsnonexclusively include those described in U.S. Pat. No. 4,942,112;quinoxaline compounds as described in U.S. Pat. No. 3,765,898; thevicinal polyketaldonyl compounds in U.S. Pat. No. 2,367,660; thealpha-carbonyls in U.S. Pat. Nos. 2,367,661 and 2,367,670; the acyloinethers in U.S. Pat. No. 2,448,828; the triarylimidazolyl dimers in U.S.Pat. No. 3,479,185; the alpha-hydrocarbon substituted aromatic acyloinsin U.S. Pat. No. 2,722,512; polynuclear quinones in U.S. Pat. Nos.2,951,758 and 3,046,127; and s-triazines in U.S. Pat. No. 4,656,272.These patents are incorporated herein by reference.

Photopolymerizable compounds end-capped with at least one epoxy,acrylate, or methacrylate group can be initiated by a free radical typephotoinitiator. Suitable free radical initiated type photoinitiatorsinclude aromatic ketones such as benzophenone, acrylated benzophenone,2-ethylanthraquinone, phenanthraquinone, 2-tert-butylanthraquinone,1,2-benzanthraquinone, 2,3-benzanthraquinone,2,3-dichloronaphthoquinone, benzyl dimethyl ketal and other aromaticketones, e.g., benzoin, benzoin ethers such as benzoin methyl ether,benzoin ethyl ether, benzoin isobutyl ether and benzoin phenyl ether,methyl benzoin, ethyl benzoin and other benzoins. Preferredphotoinitiators are 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184),benzoin, benzoin ethyl ether, benzoin isopropyl ether, benzophenone,2,2-dimethoxy-2-phenylacetophenone (commercially available fromCIBA-GEIGY Corp. as Irgacure 651), α,α-diethyloxy acetophenone,α,α-dimethyloxy-α-hydroxy acetophenone (Darocur 1173),1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-propan-1-one (Darocur2959), 2-methyl-1-[4-methylthio)phenyl]-2-morpholino-propan-1-one(Irgacure 907),2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (Irgacure369), poly{1-[4-(1-methylvinyl)phenyl]-2-hydroxy-2-methyl-propan-1-one}(Esacure KIP), [4-(4-methylphenylthio)-phenyl]phenylmethanone(Quantacure BMS), di-campherquinone. The most preferred photoinitiatorsare those which tend not to yellow upon irradiation. Suchphotoinitiators include benzodimethyl ketal (Irgacure 651),2-hydroxy-2-methyl-1-phenyl-propan-1-one (commercially available fromCiba-Geigy Corporation under the name Darocur 1173),1-hydroxy-cyclohexyl-phenyl ketone (Irgacure-184), and1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-propan-1-one (Darocur2959).

Photopolymerizable compounds end-capped with at least one vinyl ethergroup can be initiated by a radical cation type photoinitiator. Suitableradical cation type photoinitiators include various compounds whichrespond to irradiation by producing acid species capable of catalyzingcationic polymerization. See Crivello, Advances in Polymer Science, 62,p. 1-48 (1984). Onium salts of Group V, VI and VII elements are statedto be the most efficient and versatile of the cationic photoinitiators.They generate strong Lewis acids which can promote cationicpolymerization. Curing of vinyl ether compositions is not limited to aparticular class of such photoinitiators, although certain types arepreferred, including onium salts based on halogens and sulfur. Morespecifically, the onium salt photoinitiators described in Crivello'sU.S. Pat. No. 4,058,400 and in particular iodonium and sulfonium saltsof BF₄ ⁻, PF₆ ⁻, SbF₆ ⁻, and SO₃CF₃ ⁻. Preferred photoinitiators aretriarylsulfonium salts, and diaryliodonium salts. Preferred anions arehexafluorophosphate and hexafluoroantimony. They are usually required inamounts from about 0.1 to about 5 wt. %. Preferred initiators include:

where X is SbF₆ ⁻ or PF₆ ⁻. Commercially available initiators includeUVI-6974 (a SbF₆ ⁻ salt) and UVI-6990 (a PF₆ ⁻ salt) supplied by UnionCarbide. Other cationic photoinitiators are defined by the formulas

where y is 1 to 18.

The free radical or radical cation generating photoinitiator is presentin each photopolymerizable composition in an amount sufficient to effectphotopolymerization of the photopolymerizable compound upon exposure tosufficient actinic radiation. The photoinitiator is generally present inan amount of from about 0.01% to about 10% by weight of the overallcomposition, or more preferably from about 0.1% to about 6% and mostpreferably from about 0.5% to about 4% by weight based on the totalweight of the composition.

Photopolymerizable compositions may include mixtures of polymerizablecompounds end-capped with at least one actinic radiation curable group,such as the above-described epoxy or ethylenically unsaturated groups,specifically acrylate, methacrylate, and vinyl ether. Vinyl ethers canreact with acrylates. Although acrylates and vinyl ethers do notordinarily react with epoxies, mixed systems of vinyl ethers, acrylates,and epoxies can form interpenetrating networks if suitablephotoinitiators are used. Accordingly, mixed systems can be used inmaking optical devices by the methods described here. Photoinitiatorsthat are suitable for use in such mixed systems are described in U.S.Pat. No. 5,510,226, the disclosure of which is incorporated herein byreference.

For more highly fluorinated multifunctional acrylates, such as thefluorinated compound L-9367 available from 3M Specialty ChemicalsDivision, St. Paul, Minn., the structure of which is shown below, apreferred photoinitiator is a fluorinated photoinitiator such as thosedescribed in U.S. Pat. Nos. Re. 35,060 and 5,391,587, the disclosures ofwhich are incorporated herein by reference. In particular, a fluorinatedphotoinitiator having the structure (IV)

and described at Example 1 of Re. 35,060, may be used. It is alsopossible to cure the fluorinated materials of Examples A through Dwithout photoinitiators through the use of electron beam curing.

It is possible to readily cure the polymerizable compounds, such asthose described in the examples below, by heating them in the presenceof a thermal type free radical polymerization initiator. While actinicradiation curing is preferred for the imagewise exposure steps describedabove, thermal curing could be used for any non-imagewise curing step.Suitable known thermal initiators include, but are not limited to,substituted or unsubstituted organic peroxides, azo compounds, pinacols,thiurams, and mixtures thereof. Examples of operable organic peroxidesinclude, but are not limited to benzoyl peroxide, p-chlorobenzoylperoxide and like diacyl peroxides; methyl ethyl ketone peroxide,cyclohexanone peroxide and like ketone peroxides; tert-butylperbenzoate, tert-butyl peroxy-2-ethylhexoate and like peresters;tert-butyl hydroperoxide, cumene hydroperoxide and like hydroperoxides;di-tert-butyl peroxide, di-sec-butyl peroxide, dicumyl peroxide and likedialkyl peroxides; and diaryl peroxides. Other suitable organic peroxideinclude 2,5-dimethyl-2,5-di(t-butylperoxy)-hexane,1,3-bis(t-butylperoxyisopropyl)benzene,1,3-bis-(cumylperoxyisopropyl)benzene, 2,4-dichlorobenzoyl peroxide,caprylyl peroxide, lauroyl peroxide, t-butyl peroxyisobutyrate,hydroxyheptyl peroxide, di-t-butyl diperphthalate, t-butyl peracetate,and 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane. The organicperoxide is added to the composition in an amount ranging from 0.01-10%,preferably 0.1-5%, by weight based on the weight of the acrylate ormethacrylate.

Suitable azo-type thermal curing initiators include2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile),(1-phenylethyl)azodiphenylmethane,2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile),dimethyl-2,2′-azobis(1-cyclohexanecarbonitrile),2-(carbamoylazo)-isobutyronitrile, 2,2′-azobis(2,4,4-trimethylpentane),2-phenylazo-2,4-dimethyl-4-methoxyvaleronitrile,2,2′-azobis(2-methylpropane) and like azo compounds.

Other additives may also be added to the photosensitive compositionsdepending on the purpose and the end use of the light sensitivecompositions. Examples of these include antioxidants, photostabilizers,volume expanders, free radical scavengers, contrast enhancers, nitronesand UV absorbers. Antioxidants include such compounds as phenols andparticularly hindered phenols including tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane (commerciallyavailable under the name Irganox 1010 from CIBA-GEIGY Corporation);sulfides; organoboron compounds; organophosphorous compounds;N,N′-hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide)(available from Ciba-Geigy under the tradename Irganox 1098).Photostabilizers and more particularly hindered amine light stabilizersthat can be used include, but are not limited to,poly[(6-morpholino-s-triazine-2,4-diyl)[2,2,6,6,-tetramethyl-4-piperidyl)imino]-hexamethylene[2,2,6,6,-tetramethyl-4-piperidyl)imino)]available from Cytec Industries under the tradename Cyasorb UV3346.Volume expanding compounds include such materials as the spiral monomersknown as Bailey's monomer. Suitable free radical scavengers includeoxygen, hindered amine light stabilizers, hindered phenols,2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO), and the like.Suitable contrast enhancers include other free radical scavengers suchas nitrones. UV absorbers include benzotriazole, hydroxybenzophenone,and the like. These additives may be included in quantities, based uponthe total weight of the composition, from about 0% to about 6%, andpreferably from about 0.1% to about 1%. Preferably all components of theoverall composition are in admixture with one another, and mostpreferably in a substantially uniform admixture.

When the radiation curable compounds described above are cured byultraviolet radiation, it is possible to shorten the curing time byadding a photosensitizer, such as benzoin, benzoin methyl ether, benzoinethyl ether, benzoin isopropyl ether, benzil (dibenzoyl), diphenyldisulfide, tetramethyl thiuram monosulfide, diacetyl,azobisisobutyronitrile, 2-methyl-anthraquinone, 2-ethyl-anthraquinone or2-tertbutyl-anthraquinone, to the monomer, oligomer, or polymercomponent or its solution. The proportion of the photosensitizer ispreferably at most 5% by weight based on the weight of the curablecompound.

As used herein “actinic radiation” is defined as light in the visible,ultraviolet or infrared regions of the spectrum, as well as electronbeam, ion or neutron beam or X-ray radiation. Actinic radiation may bein the form of incoherent light or coherent light, for example, lightfrom a laser. Sources of actinic light, and exposure procedures, times,wavelengths and intensities may vary widely depending on the desireddegree of polymerization, the index of refraction of the photopolymerand other factors known to those of ordinary skill in the art. Suchconventional photopolymerization processes and their operationalparameters are well known in the art. Sources of actinic radiation andthe wavelength of the radiation may vary widely, and any conventionalwavelength and source can be used. It is preferable that thephotochemical excitation be carried out with relatively short wavelength(or high energy) radiation so that exposure to radiation normallyencountered before processing, e.g., room lights will not prematurelypolymerize the polymerizable material. Alternatively, the processing canutilize a multiphoton process initiated by a high intensity source ofactinic radiation such as a laser. Thus, exposure to ultraviolet light(300-400 nm wavelength) is convenient. Also, exposure by deepultraviolet light (190-300 nm wavelength) is useful. Convenient sourcesare high pressure xenon or mercury-xenon arc lamps fitted withappropriate optical filters to select the desired wavelengths forprocessing. Also, short wavelength coherent radiation is useful for thepractice of this invention. An argon ion laser operating in the UV modeat several wavelengths near 350 nm is desirable. Also, afrequency-doubled argon ion laser with output near 257 nm wavelength ishighly desirable. Electron beam or ion beam excitation may also beutilized. Typical exposure times normally vary from a few tenths ofseconds to about several minutes depending on the actinic source.Temperatures usually range from about 10° C. to about 60° C., however,room temperature is preferred.

Control of the spatial profile of the actinic radiation, that is, whereit falls on the layer of photopolymerizable material may be achieved byconventional methods. For example, in one conventional method, a maskbearing the desired light transmissive pattern is placed between thesource of actinic radiation and the photopolymerizable composition film.The mask has transparent and opaque regions which allow the radiation tofall only on the desired regions of the film surface. Masked exposure ofthin films is well known in the art and may include contact, proximityand projection techniques for printing the light transmissive patternonto the film. Another conventional method of spatial control is to usea source of actinic radiation which comprises a directed or focused beamsuch as a laser or electron beam. Such a beam intersects only a smallarea of the photo-polymerizable material film surface. The pattern ofthe desired light transmissive regions is achieved by moving this smallintersection point around on the film surface either by scanning thebeam in space or by moving the substrate so that the intersection pointis changed relative to a stationary beam. These types of exposure usinga beam source are known in the art as direct-write methods. By choosingthe spatial characteristics of irradiation, it is possible to createlight transmissive regions on the surface of the substrate and produceslab and channel waveguides. A slab waveguide is one in which theoptical wave is confined only to the plane of the film. A channelwaveguide is one in which the optical wave is also confined laterallywithin the film. A channel structure is necessary for many nonlinear andelectro-optic devices because it allows the light to be directed tocertain areas of the substrate as well as providing a mechanism forsplitting, combining optical waves, coupling light from the waveguide tooptical fibers, and maintaining the high intensity available in anoptical fiber.

The method of this invention can be used for making a wide variety ofoptical elements. By using a suitable mask and by controlling the degreeof collimation of the actinic radiation used for exposure, it is alsopossible to create arrays of micro-optical elements such as lenses orprisms which can be designed to transmit light in a direction roughlyorthogonal to the substrate. Such optical element arrays find utility inapplication to backlights, e.g., for liquid crystal displays, projectionsystems, front or rear projection screens, diffusers, collimators,liquid crystal viewing screens, light directing arrays for collimatorsand lighting fixtures, exit signs, displays, viewing screens, displaysfor projection systems, and the like. For such applications it isimportant to create an essentially cosmetically perfect device composedof individual elements which have sharp definition and smooth walls. Thecomposition of the current invention can be used to enhance the criticalaspects of definition and wall smoothness. For some applications, thesubstrate may optionally be removed from the waveguide core andcladding.

The optical elements produced by the instant invention preferably havean optical loss at 1550 nm of about 0.1 dB/cm or less to about 0.5dB/cm, more preferably less than about 0.3 dB/cm, even more preferablyless than about 0.25 dB/cm, and most preferably less than about 0.20dB/cm. In addition, the polymerized cladding, core and buffer layerspreferably have a Gardner index as described by ASTM D 1544-80 of about3 or less, more preferably about 2 or less and most preferably about 1or less.

Device testing and modeling suggest a device lifetime (time for 0.1dB/cm loss) of more than 10 years at 120° C. (operation temperature) andmore than 1 hour at 250° C. (a typical device packaging temperature),thus allowing for use of devices made in accordance with this disclosureapplicable in the aerospace, military, and telecommunicationsindustries. Flexibility of the materials allows for fabrication ofdevices with desired bending angles. Cracking is also avoided even whenthe device is exposed to very high or very low temperatures. Goodadhesion of the materials permits fabrication of robust devices on avariety of substrates without delamination even in some harshenvironments such as high temperature and high humidity. Compatibilityof device fabrication techniques with those of the semiconductorindustry allows for development of hybrid optoelectronic circuitry.

The following non-limiting examples serve to illustrate the invention.It will be appreciated that variations in proportions and alternativesin elements of the components of the photosensitive coating compositionwill be apparent to those skilled in the art and are within the scope ofthe present invention.

EXAMPLES

To synthesize the crosslinked photopolymers, the monomers or theoligomers were mixed with the photoinitiators and the antioxidant andwell stirred. The solutions obtained were coated into thin liquid filmsby spin coating, slot coating or direct liquid casting with appropriatespacers. The thickness of the film was controlled by spinning speed orspacer thickness. The thickness of the films below 50 μm was measuredwith a Sloan Dektak IIA profilometer and the thickness of the thickerfilms were measured with a microscope.

Some of the fluorinated acrylates and methacrylates used in the examplesof this invention are commercially available. For example, thefluorinated acrylates used in Examples C and D are available from 3MSpecialty Chemicals Division, St. Paul, Minn. Alternatively, thefluorinated acrylates useful in this invention can be made fromcommercially available fluorinated polyols using methods generally knownto those skilled in the art. The fluorinated polyol used in Example A,for example, is available from Ausimont USA, Inc., of Thorofare, N.J.Fluorinated acrylates can also be prepared from the polyol2,2,3,3,4,4,5,5,-octafluoro-1,6-hexanediol available from LancasterSynthesis, Inc., of Windham, N. H.

If the polymerizable compounds, such as acrylates, are synthesized frompolyols, care should be taken to remove as much as practicable anyresidual alcohols or other hydroxyl group-bearing impurities since thehydroxyl group absorbs strongly in the spectral region of interest intelecommunications device applications, namely, in the 1300 to 1550 nmregion. A preferred product purification technique is described inExample A.

Example A

A three-neck glass flask was fitted with a condenser and stirrer.Fluorolink® T brand fluorinated polyol (compound V, 900 g) andp-methoxyphenol (0.5 g) were added to the flask. The fluorinated polyolused in this example is a compound that can be described as havingstructure (V):

where the ratio m/n preferably varies from about 0.5 to about 1.4, m(average) varies from about 6.45 to about 18.34, and n (average) variesfrom about 5.94 to about 13.93. Most preferably, the ratio m/n is about1 and m (average) and n (average) are each about 10.3.

Acryloyl chloride (170 g) was then added and the mixture was vigorouslystirred. The resulting exotherm brought the temperature up to 70° C. Thetemperature was then raised to 90° C. and the reaction was run for threehours. The system was then placed under vacuum to remove the HClgenerated by the reaction and the excess acryloyl chloride. The mixturewas then cooled to room temperature. The infrared spectrum of the batchconfirmed the disappearance of the broad absorbence at 3500 cm⁻¹, whichis attributed to hydroxyl groups on the polyol. Triethylamine (124 g)was then slowly added to the reaction flask over a ½-hour period. Thesample was then filtered to remove triethyl amine hydrochloride whichformed. The sample was then washed twice with water. The resultingtetraacrylate was isolated. The tetraacrylate product is a compound thatcan be described as having structure (VI):

where the ratio m/n preferably varies from about 0.5 to about 1.4, m(average) varies from about 6.45 to about 18.34, and n (average) variesfrom about 5.94 to about 13.93. Most preferably, the ratio m/n is about1 and m (average) and n (average) are each about 10.3.

Such compounds having structure (VI) are perfluoropolyethertetraacrylates. Because they are tetra-functional, they can also beuseful in adjusting the crosslink density of the cured film to vary itsphysical properties. High molecular weight versions of this material canalso be very low in loss while tending to have better solubility thansome other materials described in this disclosure. Physical propertiesfor one of these materials are shown in the table below.

Liquid Curved Molecular Refractive Refractive # of Weight Index^(a)Index^(b) Density Hydrogens C_(H) ^(c) 2400 1.3362 1.335 1.663 26 18.0^(a)n_(D) ²⁰ ^(b)Metricon 2010 prism coupler reading at 1550 nm for acured film made using 1% photoinitiator. ^(c)Molar concentration ofhydrogen atoms in compound (described above)

Example B

Suitable monomers for use in this invention includepolydifluoromethylene diacrylates having the generic structure:CH₂═CHCO₂CH₂(CF₂)_(n)CH₂O₂CCH═CH₂ where n is preferably 1-10. For thisclass of materials, the higher the value of n, the lower the refractiveindex, the lower the crosslink density, and the lower the absorbance.These materials tend to produce relatively hard films of high cross-linkdensity. They also have excellent adhesive properties but have higherabsorption losses than some of the other materials described in thisapplication. The table below shows selected physical property values oftwo of these materials.

# of Liquid Cured Mol- Repeat Refractive Refractive # of ecular Units(n) Index^(a) Index^(b) Density Hydrogens Weight C_(H) ^(c) 4 1.39201.4180 1.433 10 370 38.7 6 1.3797 1.4061 1.510 10 370 32.1 ^(a)n_(D) ²⁰^(b)Metricon 2010 prism coupler reading at 1550 nm for a cured film madeusing 1% photoinitiator. ^(c)Molar concentration of hydrogen atoms incompound (described above)

The compound octafluorohexanediol diacrylate was made as follows. Athree-neck glass flask was fitted with a condenser. The polyol2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (OFHD, 300 g) obtained fromLancaster Synthesis of Windham, N. H., and p-methoxyphenol (0.5 g) wereadded to the flask. The flask was heated to 70° C. to melt the OFHD.Acrylol chloride (228 g) was then added and the mixture was vigorouslystirred. The resulting exotherm brought the temperature up to 90° C. Thetemperature was then held at 90° C. and the reaction was run for threehours. The system was then placed under vacuum to remove the HClgenerated by the reaction and the excess acryloyl chloride. The mixturewas then cooled to room temperature. The infrared spectrum of the batchconfirmed the disappearance of the broad absorbance at 3500 cm⁻¹, whichis attributed to hydroxyl groups on the polyol. Triethylamine (189 g)was then slowly added to the reaction flask over a ½-hour period. Thesample was then filtered to remove the triethyl amine hydrochloridewhich formed. The sample was then washed twice with water. The remainingwater was then stripped away under vacuum.

The reaction forming the octafluorohexanediol diacrylate compound (VIII)from the polyol 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (compound VII)is depicted below:

Example C

Another multifunctional acrylate that can be used in this inventioninclude the fluorinated acrylate

CH₂═CHCO₂CH₂CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)CH₂O₂CCH═CH₂

having the trade name L-12043 available from 3M Specialty ChemicalsDivision.

Example D

Another multifunctional acrylate that can be used in this inventioninclude the fluorinated acrylate

CH₂═CHCO₂CH₂(CF₂CF₂O)_(m)(CF₂O)_(n)CF₂CH₂O₂CCH═CH₂

having the trade name L-9367 also available from 3M Specialty ChemicalsDivision.

Polymerizable monomers useful in practicing the invention can also bemade from amino-terminated poly(perfluoroalkylene oxides), such asstructure IX,

HOCH₂CH₂(CH₃)NCO—CF₂O—(CF₂CF₂O)_(m)(CF₂O)_(n)—CF₂—CON(CH₃)CH₂CH₂OH  (IX)

or from the diamine of structure X,

H₂NCH₂CF₂O—(CF₂CF₂O)_(m)(CF₂O)_(n)—CF₂—CH₂NH₂  (X)

by reaction with an acrylic acid halide or anhydride in the presence ofa tertiary amine.

In order to make suitable planar polymeric optical waveguides, it ispreferred to finely control the refractive index of various core andcladding layers. While this can theoretically be achieved by tailoringthe structure of a single monomer, oligomer, or polymer component usedin a particular coating layer to achieve the desired refractive index,in practice, it is oftentimes easier to blend several monomers,oligomers, or polymer components of different refractive indicestogether to obtain the desired composite refractive index.

The refractive index of each of the polymerizable compounds made inExample A-B, or described above at Examples C-D, was measured by mixingeach with 1% by weight of an appropriate photoinitiator. The mixtureswere then spin coated onto a silicon wafer at a thickness of 5 to 10microns. The samples were purged with nitrogen and cured to a hardenedfilm with UV light. The refractive index of the films was then measuredusing a Metricon 2010 testing apparatus with a 1550 nm laser source inthe TE mode. The results are tabulated in Table 2.

TABLE 2 Sample Refractive index at 1550 nm A 1.3519 B 1.4183 C 1.3454 D1.3079

The samples were purged with nitrogen to remove oxygen, a knownphotopolymerization inhibitor, from the samples before photoinitiation.Alternatively, the container holding the samples can be evacuated toremove oxygen. Oxygen inhibition is generally not desired so that thepolymerizable materials are substantially fully cured to produce curedmaterials having refractive index values that do not drift significantlyover time or upon possible subsequent exposure to additional radiation.If desired, however, layers may be partially cured and, once the entiremulti-layer structure is built, some or all layers may be cured furtherin a post-cure exposure step, as discussed above.

Using various mixtures of the Example A-D materials, it is possible toachieve a layer with a controlled refractive index lying between 1.3079and 1.4183. It is also possible to extend this range further by usingother materials that meet the chemical structure (III) defined above.Structures with R_(f) groups that are larger or smaller than those inExamples A-D defined by Table 2 are likely to have refractive indexvalues outside the range.

It is also possible to blend the monomers satisfying generic formula(III) with other monomers, such as the non-fluorinated compoundsdescribed above. Conventional (meth)acrylates, including non-fluorinatedcompounds, can have refractive index values ranging from about 1.4346 toabout 1.5577, as shown in Table 3. The table lists refractive indexvalues of various acrylate and methacrylate monomers provided by theSartomer Company, of Exton, Pa. It is likely, however, that mixedsystems including non-fluorinated monomers will be higher in loss thanfully fluorinated systems.

TABLE 3 Sartomer Refractive Chemical Name Product index IsooctylAcrylate SR-440 1.4346 2-2(Ethoxyethoxy)ethyl Acrylate SR-256 1.4355 2(2-Ethoxyethoxy) Ethylacrylate SR-256 1.4366 Triethylene GlycolDiacetate SR-322 1.4370 Isodecyl Acrylate SR-395 1.4395 IsodecylMethacrylate SR-242 1.4414 Lauryl Acrylate SR-335 1.4416 LaurylMethacrylate SR-313 1.4420 Isodecyl Acrylate SR-395 1.4431 PropoxylatedNeopentyl Glycol Diacrylate SR-9003 1.4464 Alkoxylated DifunctionalAcrylate Ester SR-9040 1.4470 Glycidyl Methacrylate SR-379 1.4470Glycidyl Methacrylate SR-379 1.4470 Propoxylated Neopentyl GlycolDiacrylate SR-9003 1.4470 Alkoxylated Difunctional Acrylate EsterSR-9040 1.4470 Tridecyl Methacrylate SR-493 1.4472 Tridecyl AcrylateSR-489 1.4474 Caprolactone Acrylate SR-495 1.4483 Tripropylene GlycolDiacrylate SR-306 1.4485 Stearyl Methacrylate SR-324 1.4485 Tris(2-Hydroxy Ethyl) isocyanurate Triacrylate SR-368 1.4489 1,3-ButyleneGlycol Dimethacrylate SR-297 1.4489 1,3-Butylene Glycol DiacrylateSR-212 1.4501 Neopentyl Glycol Diacrylate SR-247 1.4503 Neopentyl GlycolDimethacrylate SR-248 1.4510 Adhesion Promoting Monofunctional AcidCD-9050 1.4513 Ester Ethylene Glycol Dimethacrylate SR-206 1.4522Alkoxylated Aliphatic Diacrylate Ester SR-9209 1.4533 1,4-ButanediolDiacrylate SR-213 1.4535 1,4-Butanediol Dimethacrylate SR-214 1.4545C14-C15 Acrylate Terminated Monomer SR-2000 1.4548 1,4-ButanediolDimethacrylate SR-214 1.4548 Tetrahydrofurfuryl Methacrylate SR-2031.4553 Hexanediol Diacrylate SR-238 1.4553 1,6-Hexanediol DimethacrylateSR-239 1.4556 1,6-Hexanediol Diacrylate SR-238 1.4560 TetrahydrofurfurylAcrylate SR-285 1.4563 Hexanediol Dimethacrylate SR-239 1.4565Propoxylated Trimethylolpropane Triacrylate SR-501 1.4567 CyclohexylAcrylate SR-208 1.4567 Highly Propoxylated Glyceryl Triacrylate SR-90211.4575 Tetrahydrofurfuryl Acrylate SR-203 1.4575 Cyclohexyl MethacrylateSR-220 1.4575 Tetrahydrofurfuryl Acrylate SR-285 1.4577 TriethyleneGlycol Dimethacrylate SR-205 1.4580 C14-C15 Methacrylate TerminatedMonomer SR-2100 1.4585 Tetraethylene Glycol Dimethacrylate SR-209 1.4587Propoxylated₃ Trimethylolpropane Triacrylate SR-492 1.4590 DiethyleneGlycol Diacrylate SR-230 1.4590 Polyethylene Glycol DimethacrylateSR-210 1.4598 Propoxylated Glyceryl Triacrylate SR-9020 1.4605Triethylene Glycol Diacrylate SR-272 1.4606 Diethylene GlycolDimethacrylate SR-231 1.4607 Highly Propoxylated Glyceryl TriacrylateSR-9021 1.4610 Propoxylated Glyceryl Triacrylate SR-9020 1.4612Tetraethylene Glycol Diacrylate SR-268 1.4621 Caprolactone AcrylateSR-495 1.4637 Polyethylene Glycol (200) Diacrylate SR-259 1.4639Polyethylene Glycol (400) Dimethacrylate SR-603 1.4645Di-trimethylolpropane Tetraacrylate SR-355 1.4654 Polyethylene Glycol(600) Dimethacrylate SR-252 1.4655 Polyethylene Glycol (400) DiacrylateSR-344 1.4655 Polyethylene Glycol (600) Dimethacrylate SR-252 1.4666Polyethylene Glycol (600) Diacrylate SR-610 1.4676 EthoxylatedTrimethylolpropane Triacrylate SR-454 1.4686 Ethoxylated₃Trimethyolopropane Triacrylate SR-454 1.4689 Ethoxylated₆Trimethylolpropane Triacrylate SR-499 1.4691 Ethoxylated₉Trimethylolpropane Triacrylate SR-502 1.4692 Adhesion PromotingTrifunctional Acid Ester CD-9051 1.4692 Ethoxylated₁₅ TrimethylolpropaneTriacrylate SR-9035 1.4695 Alkoxylated Trifunctional Acrylate EsterSR-9008 1.4696 Ethoxylated Trimethylolpropane Triacrylate SR-9035 1.4697Ethoxylated₂₀ Trimethylolpropane Triacrylate SR-415 1.4699Trimethylolpropane Trimethacrylate SR-350 1.4701 EthoxylatedTrimethylolpropane Triacrylate SR-415 1.4705 Ethoxylated PentaerythritolTriacrylate SR-494 1.4711 Isobornyl Acrylate SR-506 1.4722Trimethylolpropane Triacrylate SR-351 1.4723 Trifunctional MethacrylateEster SR-9010 1.4723 Trifunctional Methacrylate Ester SR-9010 1.4723Trifunctional Methacrylate Ester SR-9011 1.4724 Isobornyl AcrylateSR-506 1.4738 Isobornyl Methacrylate SR-423 1.4738 IsobornylMethacrylate SR-423 1.4740 Saret Crosslinking Agent (Trifunctional)SARET 500 1.4751 Sarit Crosslinking Agent (Trifunctional) SR-500 1.4751Di-Trimethylolpropane Tetraacrylate SR-355 1.4758 Aromatic AcidMethacrylate Half Ester in Tri- SB-600 1.4767 functional MethacrylateMonomer Pentaerythritol Triacrylate SR-444 1.4790 Aliphatic UrethaneAcrylate CN-965 1.4800 Pentaerythritol Triacrylate SR-444 1.4801Aromatic Urethane Acrylate CN-972 1.4810 Aliphatic Urethane AcrylateCN-962 1.4812 Low Viscosity Aliphatic Diacrylate Oligomer CN-132 1.4817Epoxidized Soy Bean Oil Acrylate CN-111 1.4821 PentaerythritolTetraacrylate SR-295 1.4823 Pentaerythritol Tetraacrylate SR-295 1.4847Dipentaerythritol Pentaacrylate SR-399 1.4885 Pentaacrylate EsterSR-9041 1.4887 Pentaerythritol Pentaacrylate SR-399 1.4889 Low ViscosityAliphatic Triacrylate Oligomer CN-133 1.4896 Pentaacrylate Ester SR-90411.4899 Aromatic Acid Methacrylate Half Ester In EEP SB-401 1.4905 EsterSolvent Highly Ethoxylated₃₀ Bisphenol A CD-9036 1.4906 DimethacrylateAliphatic Urethane Acrylate CN-981 1.4916 Aromatic Acid MethacrylateHalf Ester in PM SB-400 1.4921 Alcohol Solvent Aliphatic UrethaneAcrylate CN-980 1.4931 Ethoxylated Nonylphenol Acrylate SR-504 1.4936Aromatic Acid Methacrylate Half Ester in SB-500E50 1.5010 SR454 AromaticAcid Acrylate Half Ester in SR454 SB-520E35 1.5022 Aromatic AcidMethacrylate Half Ester in SB-500K60 1.5029 SR344 PhenoxyethylMethacrylate SR-340 1.5100 2-Phenoxyethyl Methacrylate SR-340 1.5109Highly Ethoxylated₁₀ Bisphenol A SR-480 1.5112 DimethacrylateEthoxylated₁₀ Bisphenol A Diacrylate SR-602 1.5142 Phenoxyethyl AcrylateSR-339 1.5151 2-Phenoxyethyl Acrylate SR-339 1.5160 Ethoxylated₆Bisphenol A Dimethacrylate CD-541 1.5227 Low Viscosity AromaticMonoacrylate CN-131 1.5259 Oligomer Stearyl Acrylate SR-257 1.5312Ethoxylated₄ Bisphenol A Dimethacrylate CD-540 1.5315 Ethoxylated₄Bisphenol A Diacrylate SR-601 1.5340 Ethoxylated Bisphenol ADimethacrylate SR-348 1.5389 Ethoxylated₂ Bisphenol A DimethacrylateSR-348 1.5424 Ethoxylated Bisphenol A Diacrylate SR-349 1.5424Ethoxylated₂ Bisphenol A Diacrylate SR-349 1.5425 Epoxy Acrylate CN-1201.5558 Epoxy Acrylate CN-104 1.5577

In addition, it is also possible to include the use of dissolvedthermoplastic materials in these formulations. The use of eitheralternative monomers and/or polymers is limited strictly by theircompatibility with the cured materials of this invention.

Comparative Example 1

A straight waveguide was made using the following procedeure. A cleansilicon wafer was silane treated by spin coating to provide an adhesivetie layer for acrylate formulations. The treated wafer was spin coatedwith a lower cladding polymerization composition including the amountsindicated of the polymerizable compounds, photoinitiator, andantioxidant listed on the table below. The thickness of the lowercladding layer was equal to or greater than about 10 μm thick. Theassembly was then cured with UV light while blanketed with nitrogen. Acore polymerizable composition was formulated including the amountsindicated of the polymerizable compounds, photoinitiator, andantioxidant set forth in the table below. The core polymerizablecomposition was then spin coated on top of the lower cladding layer. Thecore polymerizable composition was formulated such that it would have ahigher refractive index than the lower cladding layer. The thickness ofthe core layer depended on the desired height of the waveguide, whichtypically ranged from about 5 to about 9 microns for single mode guides.The core polymerizable composition was then exposed to UV light througha photomask. The unexposed material was then removed by solvent. Anupper cladding layer, which was typically made from the same materialused in the lower cladding layer, was then coated on top of the corelayer. The preferred method of coating was spin coating. The uppercladding composition was then cured.

Comparative Example 1 Ingredient or Property Core Cladding wt % SartomerSR349 10.0 wt. % — Sartomer SR238 5.0 wt. % — Sartomer SR610 27.6 wt. %32.6 wt. % Sartomer SR306 55.1 wt. % 65.2 wt. % Irgacure 651photoinitiator 1.0 wt. % 1.0 wt. % Irganox 1010 antioxidant 0.3 wt. %0.3 wt. % Refractive Index (at 1550 nm) 1.4980 1.4928 T_(g)(° C.) 11 —

Example E

The procedure used for making the Comparative Example 1 optical elementwas repeated using the formulations listed in the following table:

Example E Ingredient or Property Core Cladding wt % Product made inExample B 13 wt. % — L-12043 available trom 3M 86 wt. % 99 wt. %Specialty Chemicals Division Photoinitiator (compound IV) 1.0 wt. % 1.0wt. % Refractive Index (at 1550 nm) 1.3562 1.3471 T_(g) (° C.) 32 (seenote 1) Note 1: The T_(g) values of the core layers were determined bydynamic mechanical analysis. The T_(g) values of the cladding layerswere not determined, but they are expected to be nearly the same as thatof the core.

Example F

The procedure used for making the Comparative Example 1 optical elementwas repeated using the formulations listed in the following table:

Example F Ingredient Core Cladding Product made in Example A 60 wt. % 30wt. % L-9367 (available from 3M 38 wt. % 68 wt. % Specialty ChemicalsDivision) Compound (IV) photoinitiator 2.0 wt. % 2.0 wt. % RefractiveIndex (at 1550 nm) 1.3249 1.3188 T_(g)(° C.) −8 (see note 1) Note 1: TheT_(g) values of the core layers were determined by dynamic mechanicalanalysis. The T_(g) values of the cladding layers were not determined,but they are expected to be nearly the same as that of the core.

Example G

A straight waveguide was made using the following procedure. Unoxidizedsilicon wafers were cleaned by the Standard Clean 1 (SC1) process.Standard Clean 1 is a well-known chemical combination that is used toclean bare silicon or a silicon wafer with thermally grown or depositedoxide. The cleaning process entailed dipping the wafers into a 1:5:1solution of ammonium hydroxide:water:30% hydrogen peroxide. Thetemperature of the solution was then raised to 70° C. for ½-hour. Thewafers were then rinsed in deionized water. The wafer was then treatedwith 3-acryloxypropyltrichloro silane (Gelest Inc., Tullytown, Pa.) byapplying it onto the wafer using a clean room swab. Excess3-acryloxypropyltrichloro silane was rinsed off with ethanol followed bya light wiping with a clean room cloth to remove particles. The waferwas then dried on a hot plate set at a surface temperature of 70° C.

The lower cladding polymerizable composition was formulated per thetable below, and filtered at 0.1 microns. A quantity (1.0 ml) of thiscomposition was applied to the wafer while it sat centered on the chuckof a spin coater (available from Cost Effective Equipment division ofBrewer Science, Inc., Rolla, Mo., USA). The material was spun to obtaina 10 micron thick layer. This entailed a 100 rpm spread for 30 secondsfollowed by a ramp at 100 rpm/sec to 750 rpm for 60 seconds. The samplewas then placed in a purge box and flooded with nitrogen for two minutesat a flow of 7.1 liters per minute. The sample was then exposed at 10.4W/cm² through a 3° diffuser using a Tamarack light source. The samplewas then reloaded onto the spin coater. The core polymerizablecomposition formulated according to the table below was then filtered asabove and 1.5 ml was dispensed onto the wafer. The wafer was then spunat a 100 rpm spread for 30 seconds followed by a ramp at 100 rpm/sec to1350 rpm for 60 seconds to yield a 6 micron thick layer. The sample wasthen placed in a vacuum bell jar and evacuated to 0.2 torr to removebubbles. The photomask was then brought in contact with the sample undervacuum and held for 1 minute. The vacuum was then released and thesample was placed in a purge box as above and exposed at 11.9 mW/cm² for20 seconds. The mask was removed and the wafer was placed again on thespinner. The sample was spun at 1100 rpm and was developed for 90seconds using 8 ml of Galden® HT110 perfluorinated ether solventobtained from Ausimont USA. The sample was then coated with an upperlayer of cladding material in the same manner as the lower claddinglayer except that the cure was for 60 seconds at 9.3 mW/cm².

Example G Ingredient or Property Core Cladding Product of Example A 49.5wt. % 55.9 wt. % Product of Example B 49.5 wt. % 43.1 wt. % Darocur 1173photoinitiator 1.0 wt. % 1.0 wt. % Refractive Index (at 1550 nm) 1.37861.3723 T_(g)(° C.) 30 (see note 1) Note 1: The T_(g) values of the corelayers were determined by dynamic mechanical analysis. The T_(g) valuesof the cladding layers were not determined, but they are expected to benearly the same as that of the core.

The cured composition Example G material exhibits low dispersion, i.e.,on the order 10⁻⁶ at 1550 nm, low birefringence (≅10⁻⁴), and highenvironmental stability.

The total loss through single mode waveguides made from differentmaterials was measured as a function of the length of the waveguide.Using these results, it was possible to determine the loss through thematerial.

Loss measurements of a waveguide made using the Example E core andcladding are shown in FIG. 30. The loss was measured through a 20 mmlong waveguide. The guide was then cleaved to produce a 15 mm guide andthe loss was re-measured. The guide was then finally cleaved again toproduce a 10 mm guide. An extrapolated point of zero loss at zero lengthwas then added to the graph. The slope of the line was determined andrecorded in decibels per centimeter (dB/cm). Table 4 tabulates theresults for each of Comparative Example 1 and Examples E-G.

TABLE 4 Sample dB/om @ 1550 nm Comparative Example 1 0.75 Example E 0.29Example F 0.19 Example G 0.24

As can be seen from the loss results for Example E, F, and G, the use offluorinated alkyl or fluorinated ether acrylates is capable of producingwaveguides with very low propagation losses compared to those ofconventional materials.

The materials from Examples E, F and G also exhibited no measurablepolarization dependence when tested using a Metricon 2010 prism couplingrefractive index measuring device in both the TE and TM modes at 1550nm. The results observed imply a refractive index difference between theTE and TM polarizations of less than 0.0001, the measurement sensitivityof the testing instrument. The results for the invention compare todifferences of 0.008 (at 1.3 μm wavelength light) for high T _(g)fluorinated polyimides, as reported in U.S. Pat. No. 5,598,501. Whilefluorinated polyimides exhibit low loss, their birefringence is a cleardisadvantage to their use. As is known in the art, a birefringentmaterial has different refractive indices depending on orientation ofthe material. Since the operation of devices, such as thermo-opticswitches, directional couplers, and the like depends on small refractiveindex differences, the operation may be different for TE and TMpolarizations in highly birefringent materials. This is generallyunacceptable since the light coming into the device will have an unknownstate of polarization. The virtual absence of polarization dependence inExamples E, F, and G indicates that these materials are capable of lowloss and can produce waveguides with minimal polarization losses andshifts.

Example H

The following procedure was performed to test the assumption that aliquid material undergoing a rapid curing process is less likely toresult in physical stress than a dried thermoplastic.

A UV-coating made solely of ethoxylated bisphenol A diacrylate (EBDA,Sartomer 349 from Sartomer Company, Exton, Pa.) with 1% photoinitiatorwas spin coated on a silicon wafer and fully cured with UV light toproduce a 10 micron thick layer. Another silicon wafer was coated withJoncryl 130 (S. C. Johnson Polymer, Racine, Wis.), an aqueous styrenatedacrylic copolymer and dried for 10 minutes at 70° C. Both materials havea glass transition temperature of 62° C. Both materials also possessboth aromatic and aliphatic chemical groups. The cured film of the EBDAis highly cross-linked, while the dried film of the Joncryl 130 isthermoplastic. One of ordinary skill would normally assume that apolymer that is highly cross-linked would be under a lot more stressthan a thermoplastic polymer. This should result in a greater differencebetween TE and TM refractive index measurements. In fact, the oppositeis true as shown below:

Before Annealing After Annealing EBDA Joncryl 130 EBDA Joncryl 130 Avg.TE 1.54518 1.53968 1.54562 1.5397 Avg. TM 1.54486 1.54020 1.54542 1.5405Difference −0.00032 0.00052 −0.0001 0.0008 ANOVA P-Value 0.32223 0.026020.2565 0.0008

The table above shows the average of 10 readings for TE and TM for bothmaterials using a Metricon 2010 Prism Coupler. The difference betweenthe average TE and TM readings was determined and an analysis ofvariance (ANOVA) was performed to determine if the difference wasstatistically significant. Before annealing, the EBDA sample had adifference between TE and TM of −0.00032, however, the high P-valueindicates that this result is not statistically significant. It isessentially below the error limits of what the experiment could measure.The Joncryl 130 material had a difference of 0.00052. Unlike the EBDAsample, this difference was highly statistically significant. Afterannealing for two hours at 70° C., the difference of TE and TM for EBDAdecreased slightly and remained statistically insignificant. The Joncryl130 material, however, actually increased in difference between TE andTM and remained statistically significant. As noted above, the Joncryl130 is a thermoplastic that does have any of the additional stress thatwould be associated with a subsequent cross-linking step. When thisexperiment was repeated with a cross-linkable, solid epoxy novalac resin(Epon SU-8, Shell Chemical, Houston, Tex.), which has been used to makeoptical waveguides, as disclosed in U.S. Pat. No. 5,054,872, thedifference between TE and TM was found to be greater than 0.001regardless of annealing conditions.

As a result of this test, liquid photocurable compositions are preferredover solid thermoplastic photocurable polymers dissolved in solvents.

Example I

Perfluoropolyether diacrylates, such as those described by the genericformula

CH₂═CHCO₂CH₂CF₂O(CF₂CF₂O)_(m)(CF₂O)_(n)CF₂CH₂O₂CCH═CH₂

may be used in practicing the invention. For these materials, the valuesof both m and n can vary considerably. Final molecular weights of thesematerials can vary between about 500 and 4000. The higher the values form and n, the lower the refractive index, the lower the crosslinkdensity, and the lower the absorption loss. As can be seen from therefractive indexes and the C_(H) values given in the table below, thesematerials can be very highly fluorinated. While it is desirable to useas much fluorination as possible for loss purposes, such highlyfluorinated materials can cause difficulty in adhesion when applyingsubsequent layers, such as electrodes. In addition, these materials haverelatively limited solubility with other less fluorinated materials. Forthe higher molecular weight varieties, fluorinated photoinitiators, suchas those described in U.S. Pat. No. 5,391,587 and Reissue U.S. Pat. No.35,060, should be used. These materials also produce extremely softfilms. Glass transition temperatures for these materials can be as lowas −90° C.

Liquid Cured Molecular Refractive Refractive # of Weight Index^(a)Index^(b) Density Hydrogens C_(H) ^(c) 1100 1.3239 1.3389 1.649 10 15.02100 1.3090 1.3079 1.749 10 8.3 ^(a)n_(D) ²⁰ ^(b)Metricon 2010 prismcoupler reading at 1550 nm for a cured film made using 1%photoinitiator. ^(c)Molar concentration of hydrogen atoms in compound(described above)

Example J

A chlorofluorodiacrylate compound having the structure

CH₂═CHCO₂CH₂CF₂(CFCICF₂)₄CH₂O₂CCH═CH₂

can be used in practicing the invention. The compound has the propertieslisted in the table below.

Liquid Cured Refractive Refractive # of Molecular Index^(a) Index^(b)Density Hydrogens Weight C_(H) ^(c) 1.4216 1.4416 1.580 10 684 23.1^(a)n_(D) ²⁰ ^(b)Metricon 2010 prism coupler reading at 1550 nm for acured film made using 1% photoinitiator. ^(c)Molar concentration ofhydrogen atoms in compound (described above)

Example K

Monofunctional fluorinated acrylates having the generic structure

CF₃(CF)_(n)(CH₂)_(m)O₂CCH═CH₂

where m is typically 1 or 2 and n can range from 0 to 10 or higher, maybe used to practice the invention. Typical property values for thematerial where n=8 and m=2 are shown in the table below. For thismaterial, the higher the value of n, the lower the refractive index,glass transition temperature, and absorption loss. As noted above, whilemonofunctional monomers can be used in the invention, there may be somelong-term outgassing or material migration of any non-reacted monomersof this type. To avoid the possibility of a monofunctional monomer nothaving at least partially reacted, higher radiation dosages for longerperiods of time may be required to assure sufficient cure of thesematerials. Such efforts are generally not required usingmulti-functional monomers.

Liquid Cured Refractive Refractive # of Molecular Index^(a) Index^(b)Density Hydrogens Weight C_(H) ^(c) 1.3387 1.3325 1.6 7 569 19.7^(a)n_(D) ²⁰ ^(b)Metricon 2010 prism coupler reading at 1550 nm for acured film made using 1% photoinitiator. ^(c)Molar concentration ofhydrogen atoms in compound (described above)

Diffraction gratings, e.g., Bragg diffraction gratings, may be writtenin partially cured planar waveguide laminates, i.e., one that is notfully cured. Such partially cured waveguide laminates may be fabricatedusing the photolithographic or reactive ion etching techniques describedin this disclosure, or by any other method that is compatible with thepreferred polymerizable compositions disclosed here. The grating iswritten in at least a partially cured waveguide core, but the gratingshould extend into the core-adjacent cladding as well.

In general, the partially cured waveguide device in which a grating canbe written should be fabricated from materials using methods thatproduce a low-loss, low-birefringence, high-performance waveguide, suchas one made in accordance with the disclosure set forth above. That is,apart from any additional factors discussed below which may beconsidered in selecting materials especially suitable for makingefficient gratings in the waveguide device, the considerations notedabove for making low loss waveguides generally should not be disregardedif possible. For example, the preferred polymerizable core and/orcladding compositions are photopolymerizable and contain at least onephotoinitiator effective for initiating the photopolymerization of eachpreferably perfluorinated photopolymerizable compound in thecompositions upon exposure to a dosage of actinic radiation effective topartially cure them.

If gratings are to be written in the waveguide, especially preferredmaterials for use in fabricating at least the core and, preferably, thecladding as well, are partially cured photopolymerizable compositionscontaining roughly equal weight proportions of at least twophotopolymerizable compounds of differing refractive index (when fullycured) and characterized further by one or more of the followingproperties: Differing functionality, polymerization rates, and moleculardiffusion rates within the partially cured polymer matrix. As explainedbelow, these properties are advantageous in writing efficient gratingsin partially cured waveguides.

A method of writing diffraction gratings in polymeric waveguides isdescribed in patent application Ser. No. 09/026,764 for “Fabrication ofDiffraction Gratings for Optical Signal Devices and Optical SignalDevices Containing the Same,” filed on Feb. 20, 1998, the disclosure ofwhich is incorporated herein by reference. In that disclosure, core andcladding waveguide structures are described as being formed in partiallycured UV curable materials. The curable compositions include at leasttwo photopolymerizable comonomers. The partially cured waveguidestructure is then exposed with additional UV light through a photomaskthat generates light and dark regions in both the core and cladding. Inthe light regions, the UV radiation causes additional polymerization ofthe monomers to occur. Because the monomers are chosen so as to havedifferent polymerization and diffusion rates, the polymer formed in thelight areas during the phase mask exposure, or “writing,” step has adifferent composition than the polymer in the dark areas. After exposurethrough the mask is complete, there remains unreacted monomer.

Without intending to be bound by or limited to any theoreticalexplanation for the mechanism at work in the invention, it is believedthat this unreacted monomer will diffuse to establish a uniform monomercomposition throughout the partially cured portions of the device. Whenthe device is subsequently uniformly exposed without a mask, all of theremaining monomer is converted to polymer. This full-cure exposure steplocks in the polymeric compositional differences between the light anddark regions and results in a permanent grating. Modulation of therefractive index in the fully cured diffraction grating arises from thisdifference in composition. As mentioned above, this process worksbecause the polymers resulting from photopolymerizable of the monomers,oligomers, or polymers selected for use in the core composition and,preferably, the cladding composition as well, differ in refractive indexand the selected monomers, oligomers, and polymers differ in cure rateand diffusion rate. It is believed that these differences cause thecomposition at a selected point in the device to vary as a function ofexposure time and radiation dosage. If the composition did not vary withexposure, regions that received more exposure through the phase maskwould be expected to have the same percentage of each monomer as thedark areas. Consequently, no diffusion would be expected to take placebetween the light and dark regions. When subsequently uniformly exposedagain to achieve full cure, both the light and dark regions would havethe same refractive index and no grating would result.

A model for explaining the creation of modulations in the refractiveindex of a planar waveguide device is shown in FIGS. 33A to FIG. 33F.For the purposes of illustration, the simplified case of a binderlesstwo monomer (A* and B*) photopolymerizable system in which thepolymerization reaction rate of monomer A* is higher than that ofmonomer B* is shown. Before exposure to the grating writing radiation,there are both species of unreacted monomer A* and monomer B* in thepartially polymerized waveguide, as shown in FIG. 33A. For simplicity,polymer A and polymer B already formed during the waveguide fabricationprocess are not shown.

The sinusoidal pattern 18 of the grating writing radiation intensity,I(x), including intensity maxima and intensity minima, is shown adjacentto the brighter regions and darker regions of the partially polymerizedwaveguide material in FIG. 33C. The grating writing radiation intensitypattern may be produced using a phase mask 19, as shown in FIG. 34, by atwo-beam interference set-up 20, as shown in FIG. 35, or by any othermethod.

Bearing in mind that the waveguide is already partially polymerized fromthe waveguide fabrication process, further polymerization of monomer A*is initiated in the brighter regions of the writing pattern. Since thepolymerization rate of monomer A* is faster than that of monomer B*,with time, the brighter regions contain primarily polymer A while thedarker regions have mainly polymer B even after removal of theinterference pattern, as shown in FIG. 33D and FIG. 33E.

The brighter regions 21 are expected to become enriched in the morequickly formed polymer (polymer A) and depleted of the more quicklyconsumed monomer (monomer A*), as shown in FIG. 33C and FIG. 33D. Due tothe resulting concentration gradients of monomer A*, monomer A* isexpected to diffuse from the darker region 22 to the brighter region inorder to establish a uniform concentration, as shown in FIG. 33D. As inany diffusion process, temperature, concentration difference, andmobility of the monomers will affect the overall diffusion rate.

After some enrichment by diffusion of the faster reacting monomer A*into the light regions and enrichment of the darker regions by theslower reacting monomer B*, the waveguide is flood exposed to react allunreacted monomer to “lock in” the concentration gradients of polymer Aand polymer B. The flood exposure taking place in FIG. 33E may beaccomplished using any fast-acting radiation source, such as an actinicradiation source suitable for the polymerizable compositions selected,such as a ultraviolet (UV) radiation source (not shown). While heatcould be applied to effect the final uniform curing step, actinicradiation is preferred due to its fast cure time in light transmissivesystems. Optionally, both a final full actinic radiation cure and afinal full heat cure can be carried out. During this step, unreactedmonomer B* is polymerized. Assuming that the refractive indices ofpolymer A and polymer B are different, a steady state or “permanent”modulation of the refractive index, i.e., a grating, is thereby formedin the waveguide. The grating has the same period as the light patterncreated by the phase mask, two interfering beams, or other form ofwriting radiation. The maximum modulation depth is given by thedifference of the indices of the individual components, as shown in FIG.33F.

While differences in refractive index, diffusion, and cure rate canproduce gratings, the need for very high grating efficiency is typicallynot achieved by these differences alone. In order to achieve an evenhigher compositional change with exposure, it has been discovered thatchoosing monomers of differing functionality can substantially boost theperformance of these gratings. Functionality in this case is defined asthe number of actinic radiation curable functional groups per monomermolecule. A wide variety of monomers having actinic radiation curable(ARC) groups could be selected. Preferred ARC groups include epoxies andethylenically unsaturated groups, such as acrylates, (meth)acrylates,and vinyl ethers, to name just a few. Other suitable reactive groups aredescribed above.

To introduce how the functionality of the monomers can effectcomposition, several conceptual Examples 1-3 will first be discussedfollowed by presentation of a preferred comonomeric composition (Example4). In each of these examples, it is assumed that the relative reactionand diffusion rates of the monomers are the same.

Example 1

A formulation of two monomers with the characteristics shown in Table 1is provided. As noted above, “functionality” is defined as the number ofactinic radiation curable groups per monomer molecule.

TABLE 1 Monomer Molecular Wt. Functionality Wt. % A 100 2 50 B 100 2 50

For a 100 g quantity of the above mixture, the values tabulated in Table2 result.

TABLE 2 Initial Relative Initial Wt. % Mon- Equivalents Weight of ofomer Moles Equivalents % Equivalents Equivalents A 0.5 1.0 50 50 50 B0.5 1.0 50 50 50

The values shown for the number of moles and the number of equivalentsare the typical values familiar to chemists and physicists. The numberof moles is merely the weight of the monomer divided by its molecularweight. The number of equivalents is the number of moles of the monomermultiplied by its functionality. When polymerization occurs, a reactivegroup from one of the monomers adds to the growing polymer chain. Thelikelihood that a particular free monomer will react is dependent on theconcentration of the reactive groups for the monomers. To determine thisconcentration at the start of the reaction, the relative amount ofequivalents of each monomer was determined as a percentage of the totalnumber of equivalents and reported in the tables as Equivalents %. Thesevalues are multiplied by the molecular weight of the respective monomersto arrive at the initial relative weight of equivalents of each of themonomers. The initial wt. % of the equivalents of the monomers is thencalculated. As can be seen in Table 2, the initial wt. % of theequivalents of the monomers in this example is the same as the wt. % ofthe monomers. Because the final wt. % of a monomer in a polymer is equalto the wt. % of the monomers, the fully polymerized polymer will in thiscase be composed of 50% of monomer A and 50% of monomer B. Based on theinitial wt. % of equivalents of the monomers, when the polymer firstbegins to form, it will also be composed of 50% of monomer A and 50% ofmonomer B. Since the reaction and diffusion rates are assumed to be thesame, this suggests that the concentration of the monomers will not varyas the polymerization proceeds. This means that this idealized materialwill not likely form a grating by the process previously described.Accordingly, such a component of monomer A and B would not be preferredfor use in making photopolymerized diffraction gratings.

Example 2

The properties of interest for two monomers A and B which differ inequivalent weight are shown in Table 3 below:

TABLE 3 Monomer Molecular Wt. Functionality Wt % A 100 2 50 B 200 2 50

For a 100 g quantity of the above mixture, the values tabulated in Table4 will result.

TABLE 4 Initial Relative Initial Wt. % Mon- Equivalents Weight of ofomer Moles Equivalents % Equivalents Equivalents A 0.5 1.0 66.67 66.6750 B 0.25 0.5 33.33 66.67 50

As can be seen in Table 4, the initial wt. % of equivalents is equal tothe wt. % of the monomers. Accordingly, no concentration gradient duringcure will be expected and no grating is expected to result.

Example 3

Monomers A and B have the same molecular weights, but they havedifferent functionalities as shown in Table 5:

TABLE 5 Monomer Molecular Wt. Functionality Wt. % A 100 2 50 B 100 3 50

For a 100 g quantity of the above mixture, the values shown in Table 6will result.

TABLE 6 Initial Relative Initial Wt. % Mon- Equivalents Weight of ofomer Moles Equivalents % Equivalents Equivalents A 0.5 1.0 0 40 40 B 0.51.5 60 60 60

As shown in Tables 5 and 6, the initial wt. % of equivalents isdifferent than the wt. % of the monomers. This implies that there willbe a concentration gradient as polymerization proceeds. Accordingly,such a combination of monomers could be expected to form a grating evenif the reaction and diffusion rates of the monomer are the same.

When the reaction first begins, there are equal numbers of molecules ofboth monomers A and B. Since monomer B has one and one-half as manyreactive groups as monomer A, the polymerization will initially use moremolecules of B then monomer A. As the reaction proceeds, theconcentration of unreacted monomer B molecules will begin to decreaseand the likelihood of monomer A molecules polymerizing will increase.Once the polymerization is complete, an equal number of both monomericmolecules will have been consumed by the reaction and the concentrationby weight of monomers in the polymer will be equal.

Example 4

Monomer A is octafluorohexanediol diacrylate obtained commercially.Monomer B is the tetra-acrylate of Fluorolink® T brand tetra-functionalfluorinated polyether polyol from Ausimont Corporation.

TABLE 7 Monomer Molecular Wt. Functionality Wt. % A 370 2 50 B 2416 4 50

For a 100 g quantity of the above mixture of the monomers A and B, thevalues shown in Table 8 are expected.

TABLE 8 Initial Relative Initial Wt. % Mon- Equivalents Weight of ofomer Moles Equivalents % Equivalents Equivalents A 0.135 0.2703 76.55283.2 33.33 B 0.021 0.0828 23.45 566.5 66.67

This set of monomers A and B should produce a grating since the valuesfor the weight percent of Table 7 and the initial weight percent ofequivalents of Table 8 for each monomer are unequal.

A Monte Carlo calculation was performed for each of the above examples.The calculation was performed using a computer program based on the flowchart shown in FIG. 40. The algorithm can be used to evaluate thepotential of a selected pair of monomers characterized in terms ofmolecular weight, functionality, and initial weight proportions in thecomposition to form a diffraction grating in waveguides.

The program begins by simulating 10,000 theoretical molecules, e.g.,monomers A and B, based on the starting formulation. Since each of themonomers in the above examples is present at the 50 wt. % level, thereare 5000 unreacted molecules each monomer at the start of thecalculation. The fraction of end groups for the monomer A is calculated.A random number between 0 and 1 is then chosen. If the random number isless than the fraction of end groups for the monomer A, then onemolecule of A is considered to have been added to the forming polymerand the number of unreacted molecules of A is decreased by one. If therandom number is greater than the fraction of end groups of A, then amolecule of B is considered to have been added to the forming polymerand the number of free molecules of B is decreased by one. The weight %of A in the forming polymer is then calculated and recorded. Thefraction of end groups for A in the remaining free monomer is thenrecalculated. The process is repeated until all of the molecules areconverted to polymer.

FIG. 41 shows the results of these calculations for each of the aboveexamples. Examples 1 and 2 initially show some deviation from the 50%level as a result of the random nature of this process. However, theyquickly approach the 50% level after only about a 1000 molecules havebeen added to the polymer. Since the actual number of molecules used inmaking a grating is much larger, such random fluctuations would havelittle impact on making an actual grating. In both Examples 3 and 4,there is some early fluctuation in the values as a result of this randomapproach, but both curves approach the 0.5 level until virtually all ofthe molecules are consumed. This calculated result demonstrates theeffectiveness of using monomers having different functionalities inproducing effective gratings.

Accordingly, a method of making diffraction gratings in a planarpolymeric waveguide laminate will now be described. A waveguide isprovided which includes a polymeric light guiding core surrounded by alower refractive index material. As noted above, the lower refractiveindex material may be a substrate, a buffer layer of a support includinga substrate, or a lower cladding layer on a substrate.

The light guiding core in which the grating is to be written should notbe fully cured prior to the grating writing step. Preferably, the coreand at least that portion of the cladding surrounding the core in whichthe grating will be written is only partially cured prior to the gratingwriting step. More preferably, the extent of cure in the waveguideformation step is minimized to allow for a maximum of extent of furtherpolymerization during the grating formation step. Doing so increases thepotential difference between the maximum and minimum refractive index inthe final grating for a given polymerizable composition.

Especially preferred polymerizable compositions for fabricating the coreand, if desired, the cladding layers as well, of waveguide laminatesintended for subsequent grating writing are those that include roughlyequal weight proportions of two or more multi-functionalphotopolymerizable monomers, oligomers, or polymeric compounds(“comonomers”) which differ in polymerization reaction rate andfunctionality. It is preferred that the functionality of the at leasttwo comonomers of the composition differ by at least one, and,preferably, by at least two. The photopolymerizable composition shouldalso include an effective amount of a suitable photoinitiator or mixtureof suitable photoinitiators.

Polymerizable compositions having, say, two comonomers of differingfunctionality should be able to form efficient diffraction gratings evenif the polymerization reaction rates of the individual monomers andtheir respective diffusion rates are the same. The increased performanceof the resulting diffraction grating is especially pronounced, however,if a monomer with a higher functionality also polymerizes at a fasterrate than a monomer with a lower functionality. If a monomer with ahigher functionality polymerizes at a slower rate than a monomer with alower functionality, then the advantage produced by the higherfunctionality will be expected to be offset somewhat.

One such suitable core composition includes roughly equal weightproportions of the low-loss low-birefringence perfluorinatedphotopolymerizable tetra-acrylate compound having structure (VI)(synthesized from Fluorolink® T brand fluorinated polyether polyol fromAusimont USA) and the perfluorinated photopolymerizable di-functionaloctafluorohexanediol diacrylate compound having structure (VIII).Synthesis of the tetra-acrylate is exemplified by Example A while thatof the di-acrylate is exemplified by Example B. A composition of the twocompounds together with a photoinitiator is exemplified by Example G.

Photo-differential scanning calorimetry studies confirm that the higherfunctionality comonomer of this system, i.e., the tetra-acrylate of theFluorolink® T fluorinated polyether polyol (curve 24), reacts at ahigher rate than the lower functionality octafluorohexanediol diacrylate(curve 23), as shown at FIG. 36.

Once the partially polymerized waveguide is made, the grating is“written” in the waveguide. This step is accomplished by exposing theinside the partially polymerized waveguide to an interference pattern ofsufficient intensity to effect additional polymerization. Theinterference pattern can be established, for example, using aconventional phase mask 19 designed for writing gratings, such as thatshown in FIG. 34, or by using a conventional two-beam interference setup20, as shown in FIG. 35.

The fabrication of gratings in a planar waveguide using a phase mask isshown schematically in FIG. 34. Light of wavelength λ illuminates thephase mask of period Λ. The writing light is diffracted by the phasemask. The intensity distribution resulting from the interference patterncreated by the phase mask at the waveguide initiates furtherphotochemical reaction in the partially cured photopolymerizablecomposition of the waveguide. The result is the creation of a phasegrating written in the waveguide with period Λ_(g). For a phase maskthat is designed to diffract in the +1 and −1 orders, the grating periodis one-half the phase mask period. Light travelling inside the waveguidegrating is reflected when its wavelength is equal to λ_(B)=2n_(eff)Λ_(g)where n_(eff) is the waveguide's effective refractive index and λ_(B) isthe Bragg wavelength.

For the creation of a purely sinusoidal pattern, it is necessary to usea phase mask with a 50% diffraction efficiency in the +1 and −1diffraction orders and 0% efficiency in the 0^(th) and all higherorders. In reality, due to phase mask fabrication errors, there isalways some small percentage of light diffracted in unwanted orders. Ifthe phase mask has as little as, say, 5% diffraction efficiency in the0^(th) order, the grating will still have a period of Λ/2, but theinterference maxima are not all at the same intensity level.

A phase mask for writing gratings is itself a grating, typically etchedin a silica substrate, with an etching depth such that it diffracts mostof the light in the +1 and −1 orders. Beams corresponding to the +1 and−1 diffraction orders are interfered inside the material where theycreate a sinusoidal interference pattern. This diffraction pattern isvery important for the quality of the grating that is formed in thematerial. Typical measured diffraction efficiencies for commerciallyavailable phase masks are 0^(th) order (η_(o)) 7.7%, 1^(st) order (η₁)42%, −1^(st) order (η⁻¹) 39.6%, 2^(nd) order (η₂) 6%, and −2^(nd) order(η⁻²) 4%.

Preferably, the waveguide sample is exactly positioned under the phasemask such that the spacing between the phase mask and the waveguide issubstantially constant across the waveguide.

Although writing using a phase mask is desirable in a manufacturingsetting, as noted above, a two-beam interference set-up can also be usedto write the grating in the partially polymerized waveguide. Thefabrication of gratings in a planar waveguide using a two-beaminterference set-up is shown schematically in FIG. 35. Light beam 23from light source 24 preferably passes through beam splitter 25 so thattwo interfering beams 26, 27, separated by angle 2θ, interfere at thepartially polymerized optical waveguide device 28. Mirrors can be usedto position the beams. The light source can be a UV laser or othersource of actinic radiation.

One advantage of the two-beam interference approach is that a sinusoidalintensity pattern in the polymerizable material is more likely than inthe phase mask approach. Another advantage is that the period of thegrating can be changed simply by changing the angle between theinterfering beams. Since each phase mask is designed for a specificilluminating wavelength and grating period, a new mask is required everytime a change in the grating period is desired.

Gratings have been written in planar waveguiding optical devicesaccording to the invention using both the phase mask and interferingbeam approach.

Following the grating writing step, the waveguide with the grating isflood exposed with actinic radiation to fully cure thephotopolymerizable layers thereby “locking in” the periodic refractiveindex variations, and prevent further material diffusion.

Example L

A grating was written in a single mode straight waveguide according tothe procedure described in patent application Ser. No. 09/026,764referred to above. The waveguide was made using a photopolymerizablecomposition including about 50 wt. % of the structure (VI)tetra-acrylate obtained from the Fluorolink® T fluorinated polyetherpolyol material from Ausimont USA and about 50 wt. % ofoctafluorohexanediol di-acrylate (structure VIII) based on the totalweight of these two compounds, and including about 1 wt. %photoinitiator. The period of the phase mask was selected to product areflection at 1550 nm. The transmission spectrum of this grating isshown in FIG. 37. The intensity of the transmitted signal at thiswavelength decreased by over 45 dB, the limit of the detection equipmentused. As demonstrated by this data, a highly efficient grating was madeusing these materials and fabrication methods.

Example L

A clean silicon wafer is used as a substrate. A liquid negative-tonephotopolymerizable composition is formulated to include 55.9 wt. % ofcompound (VI) (the tetra-acrylate of the Fluorolink® T brand fluorinatedpolyether polyol made according to the procedure of Example A), 43.1 wt.% of octafluorohexanediol diacrylate compound (VIII) made according tothe procedure of Example B, and 1 wt. % Darocur 1173 photoinitiator toform a cladding polymerizable composition. The cladding composition isspin-coated on the substrate to form a lower cladding coating that is 10microns thick. The lower cladding coating is then uniformly exposed toultraviolet light under a mercury lamp (Hg line wavelength =365 nm) toform a solid thin film of refractive index 1.3723 (at 1550 nm when fullycured) as a lower cladding layer. The exposure time is kept short (1sec.) at this point to obtain a layer that is only partiallypolymerized.

A liquid negative-tone photopolymerizable composition is formulated toinclude 49.5 wt. % of compound (VI), 49.5 wt. % of compound (VIII) madeaccording to the procedure of Example B, and 1 wt. % Darocur 1173photoinitiator to form a core polymerizable composition. The corecomposition has a refractive index of 1.3786 (at 1550 nm when fullycured). The core composition is spin-coated on the lower cladding layerto form a core coating that is 6 microns thick. The core coating isplaced in contact with a photoimaging mask where the waveguiding circuit(a cascaded 4-channel add/drop device where each of the four add/dropelements in the cascade is a Mach-Zehnder interferometer) is clear (thewidth of the waveguides in the mask is 6 microns). The core coating isselectively UV-cured through the mask under the mercury lamp for a shorttime of 3 sec. to ensure only partial polymerization. The mask isremoved and the unexposed sections are developed away using anappropriate solvent.

Additional cladding composition as listed above is formulated andspin-coated onto the core structure so as to form a conformal layer thatis 10 microns thick and that layer is subsequently blanket UV-exposedunder the mercury lamp to form a solid conformal film of refractiveindex 1.3723 (at 1550 nm when fully cured) as an overcladding layer.This layer is also exposed for a short time (1 sec.) to ensure onlypartial polymerization at this stage. A phase mask with a grating isused to print (using an Argon ion laser operating at 363.8 nm) a gratingacross the core in each of the four Mach-Zehnder devices. The samplewith the planar waveguiding circuit is held parallel to the phase masksat 50 microns from the mask. The laser beam is directed perpendicularlyto the mask and the sample. The laser beam diameter is 3 mm (at 1/e²intensity). The laser is scanned 3 mm across the center of the 6-mm-longMach-Zehnder arms, creating gratings in the three partially curedwaveguide layers. The sample is finally subjected to a final UV cure ina nitrogen ambient atmosphere under the mercury lamp (60 sec.) and afinal thermal cure (90 deg. C. for 1 h) is carried out to effect a fullpolymerization of all three layers. Testing of the sample reveals thatall the gratings are reflecting the desired wavelength channels.

Compositions made from the same two comonomers in approximately the sameproportions as that made in Example G and Example L have very desirablethermo-optic properties after curing. The rate of change in therefractive index of the cured composition with temperature, dn/dt, isapproximately −3×10⁻⁴/° C. This property results in a tuning rate ofabout −0.256 nm/° C. for gratings made from this material, as shown bythe graph appearing in FIG. 38. Importantly, the curve is remarkablylinear which permits highly predictable and reproducible tuning of thereflected wavelength.

While this property of linear tunability is a highly desired property inmaking thermo-optic devices, it also useful in making gratings which arestable to temperature changes. This can be accomplished by changing thesubstrate on which the grating is made. By choosing substrates withdifferent coefficients of thermal expansion (CTE), the expansion rate ofthe Bragg grating can be altered. The change in the Bragg wavelength ofthe grating with temperature (dλ_(B)/dt), as shown in FIG. 39, can bealtered by using substrates with different CTEs. Substrates that producea value of dλ_(B)/dt as little as −0.06 nm/° C. have been developed.Datum 30 refers to the urethane-coated polycarbonate substrate notedabove.

Gratings made from the octafluorohexanediol di-acrylate/tetra-acrylateof Fluorolink® T material in accordance with the invention showed aBragg wavelength shift of just 0.2 nm when the ambient relative humiditywas changed by 90% at a constant temperature of 50° C. This result wasfavorably much smaller than the result obtained using gratings made fromother materials where the shift was 3.7 nm. This unexpected benefit mayallow optical devices made in accordance with the invention to bepackaged without having to be hermetically sealed.

It will be apparent to one skilled in the art that the manner of makingand using the claimed invention has been adequately disclosed in theabove-written description of the preferred embodiment(s) taken togetherwith the drawings; and that the above described preferred embodiment(s)of the present invention are susceptible to various modifications,changes, and adaptations, and the same are intended to be comprehendedwithin the meaning and range of equivalents of the appended claims.

Further, although a number of equivalent components may have beenmentioned herein which could be used in place of the componentsillustrated and described with reference to the preferred embodiment(s),this is not meant to be an exhaustive treatment of all the possibleequivalents, nor to limit the invention defined by the claims to anyparticular equivalent or combination thereof. A person skilled in theart would realize that there may be other equivalent componentspresently known, or to be developed, which could be used within thespirit and scope of the invention defined by the claims.

We claim:
 1. A method for making an optical element having a support anda light-transmissive patterned core thereon, the light-transmissivepatterned core having a refractive index, the support defining acore-interfacing surface having a refractive index, the methodcomprising the steps of: applying a photopolymerizable core compositionto the support to form a photopolymerizable core composition layer, thephotopolymerizable core composition including at least onephotoinitiator and at least one photopolymerizable monomer, oligomer, orpolymer having at least one photopolymerizable group, thephotopolymerizable monomer, oligomer, or polymer including aperfluorinated substituent, the perfluorinated substituent selected fromthe group consisting of —(CF₂)_(x)—,—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—, and—CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)—, where x is a number 1 through10, m designates the number of randomly distributed perfluoroethyleneoxybackbone repeating units, n designates the number of randomlydistributed perfluoromethyleneoxy backbone repeating subunits, and pdesignates the number of —CF(CF₃)CF₂O— backbone repeating subunits;imagewise exposing an imaged portion of the photopolymerizable corecomposition layer to sufficient actinic radiation to at least partiallypolymerize the imaged portion and to define a non-imaged portion of thephotopolymerizable core composition layer; removing the non-imagedportion without removing the imaged portion to form thelight-transmissive patterned core from the imaged portion; applying apolymerizable upper cladding composition onto the light transmissivepatterned core to form an upper cladding layer; and at least partiallycuring the upper cladding layer, such that the refractive index of theupper cladding layer and the refractive index of the core-interfacingsurface are both lower than the refractive index of thelight-transmissive patterned core where adjacent one another.
 2. Themethod of claim 1 wherein the perfluorinated substitutent is—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂— and the ratio m/n varies from about0.5 to about 1.4.
 3. The method of claim 2 wherein the ratio m/n isabout 1 and the molecular weight of the photopolymerizable monomer,oligomer, or polymer is between about 2000 and about
 2800. 4. The methodof claim 1 wherein the photopolymerizable group is an epoxy unsaturatedgroup or an ethylenically unsaturated group.
 5. The method of claim 4wherein the epoxy group is selected from the group consisting of:


6. The method of claim 4 wherein the ethylenically unsaturated group isselected from a group consisting of vinyl ethers, acrylates, andmethacrylates.
 7. The method of claim 1 wherein the photopolymerizablemonomer, oligomer, or polymer has the structure A—R—R_(f)—R′—A where Rand R′ are divalent or trivalent connecting groups selected from a groupconsisting of alkyl, aromatic, ester, ether, amide, amine, or isocyanategroups, the photopolymerizable group, A, is selected from the groupconsisting of

 where Y=H or D, and X=H, D, F, Cl or CH₃; and the perfluorinatedsubstitutent is R_(f).
 8. The method of claim 7 wherein the connectinggroup R is selected from a group consisting of —CH₂— or—CH₂C(A)HCH₂OCH₂—, and the connecting group R′ is selected from a groupconsisting of —CH₂— or —CH₂OCH₂C(A)HCH₂—.
 9. The method of claim 1wherein the polymerizable upper cladding composition includes at leastone photoinitiator and at least one photopolymerizable monomer,oligomer, or polymer having a photopolymerizable group, saidphotopolymerizable monomer, oligomer, or polymer including aperfluorinated substituent.
 10. The method of claim 1 wherein thesupport includes a silicon wafer substrate.
 11. The method of claim 1where the support is a laminate formed by: applying a coating of apolymerizable lower cladding composition to the substrate, thepolymerizable lower cladding composition including at least onephotoinitiator and at least one photopolymerizable monomer, oligomer, orpolymer having at least one photopolymerizable group, thephotopolymerizable monomer, oligomer, or polymer including aperfluorinated substituent; and at least partially curing thepolymerizable lower cladding composition to form a lower cladding layer.12. The method of claim 11 wherein the step of at least partially curingthe polymerizable lower cladding composition includes exposing thepolymerizable lower cladding composition to heat or actinic radiation orboth.
 13. The method of claim 1 wherein the photopolymerizable corecomposition includes a first photopolymerizable monomer, oligomer, orpolymer compound and a second photopolymerizable monomer, oligomer, orpolymer compound each including at least two photopolymerizable groupsand a perfluorinated substituent.
 14. The method of claim 13 wherein thefirst photopolymerizable monomer, oligomer, or polymer compound has afirst functionality and the second photopolymerizable monomer, oligomer,or polymer compound has a second functionality, the difference betweenthe first functionality and the second functionality being at least one.15. The method of claim 14 wherein the second photopolymerizablemonomer, oligomer, or polymer compound is a tetra-functional or higherfunctionality compound, and the first photopolymerizable monomer,oligomer, or polymer compound is a di-functional or higher functionalitycompound.
 16. The method of claim 15 wherein the firstphotopolymerizable monomer, oligomer, or polymer compound is adi-acrylate compound, and the second photopolymerizable monomer,oligomer, or polymer compound is a tetra-acrylate compound.
 17. Themethod of claim 14 wherein the photopolymerizable core compositioncomprises: from about 40 wt. % to about 60 wt. % of the firstphotopolymerizable monomer, oligomer, or polymer compound and from about40 wt. % to about 60 wt. % of the second photopolymerizable monomer,oligomer, or polymer compound based on the weight of thephotopolymerizable core composition.
 18. The method of claim 17 whereinthe photopolymerizable core composition comprises: about 50 wt. % of thefirst photopolymerizable monomer, oligomer, or polymer compound andabout 50 wt. % of the second photopolymerizable monomer, oligomer, orpolymer compound based on the total weight of the firstphotopolymerizable monomer, oligomer, or polymer compound and the secondphotopolymerizable monomer, oligomer, or polymer compound.
 19. A methodfor making an optical element comprising: providing a support; applyinga photopolymerizable composition to the support to form aphotopolymerizable layer, the photopolymerizable composition includingan effective amount of at least one photoinitiator to initiate aphotopolymerization reaction and at least one photopolymerizablemonomer, oligomer, or polymer having at least one photopolymerizablegroup, the photopolymerizable monomer, oligomer, or polymer including aperfluorinated substituent, the perfluorinated substituent beingselected from a group consisting of —(CF₂)_(x)—,—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—, and—CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)—, where x is a number from 1through 10, m designates the number of randomly distributedperfluoroethyleneoxy backbone repeating units, n designates the numberof randomly distributed perfluoromethyleneoxy backbone repeatingsubunits, and p designates the number of —CF(CF₃)CF₂O— backbonerepeating subunits; at least partially curing the photopolymerizablelayer; forming a waveguide core region in the photopolymerizable layerby a method selected from a group consisting of reactive ion etching,micro replication, direct laser writing, and laser ablation; applying apolymerizable composition onto the waveguide core to form an uppercladding; and at least partially curing the polymerizable compositionforming the upper cladding.
 20. The method of claim 19 wherein the stepof forming the waveguide core includes: protecting a region of thephotopolymerizable layer with a reactive ion etching-resistant material,the region thus defining an unprotected region; and removing theunprotected region of the photopolymerizable layer to form the waveguidecore, the waveguide core being a raised rib core.
 21. The method ofclaim 19 wherein the step of forming the core includes: protecting aregion of the layer with a reactive ion etching-resistant material; andremoving unprotected regions of the at least partially cured layer toform a trench in the lower cladding layer.
 22. The method of claim 21further comprising the steps of: applying a photopolymerizable corecomposition to the trench; and at least partially curing thephotopolymerizable core composition.
 23. The method of claim 19 whereinthe photopolymerizable composition is applied to an at least partiallycured lower cladding layer in contact with the support.
 24. The methodof claim 19 wherein the photopolymerizable composition is applied indirect contact with the support.
 25. The method of claim 19 furthercomprising the step of: applying an electrode to the upper cladding inalignment with the core.
 26. The method of claim 19 wherein thephotopolymerizable monomer, oligomer, or polymer has the structureA—R—R_(f)—R′—A where R and R′ are divalent or trivalent connectinggroups selected from the group consisting of alkyl, aromatic, ester,ether, amide, amine, or isocyanate groups; the polymerizable group, A,is selected from the group consisting of

 CY₂═C(X)COO—, and CH₂═CHO—;  where Y═H or D, and X═H, D, F, Cl or CH₃;and the perfluorinated substitutent is R_(f).
 27. The method of claim 26wherein the connecting group R is selected from a group consisting of—CH₂— or —CH₂C(A)HCH₂OCH₂— and the connecting group R′ is selected froma group consisting of —CH₂— or —CH₂OCH₂C(A)HCH₂—.
 28. A waveguidegrating made from a composition comprising: a first photocurablemultifunctional perfluorinated compound having a first functionality; asecond photocurable multifunctional perfluorinated compound having asecond functionality, the difference between the second functionalityand the first functionality being at least one, the first photocurablemultifunctional perfluorinated compound and the second photocurablemultifunctional perfluorinated compound each include a perfluorinatedsubstituent selected from a group consisting of—CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—, and—CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)—, where x is 1-10, m designatesthe number of perfluoroethyleneoxy backbone repeating units, ndesignates the number of perfluoromethyleneoxy backbone repeatingsubunits, and p designates the number of —CF(CF₃)CF₂O— backbonerepeating subunits; and a photoinitiator in an amount effective toinitiate a photopolymerization reaction.
 29. A method for making anoptical element having a support layer and a core layer, the core layerdefining a light-transmissive patterned region, the method comprisingthe steps of: applying a photopolymerizable composition to the supportto form the core layer, the photopolymerizable composition including atleast one photoinitiator and at least one monomer, oligomer, or polymerhaving at least one photopolymerizable group, the monomer, oligomer, orpolymer including a perfluorinated substituent selected from the groupconsisting of —(CF₂)_(x)—, —CF₂O—[(CF₂CF₂O)_(m)(CF₂O)_(n)]—CF₂—, and—CF(CF₃)O(CF₂)₄O[CF(CF₃)CF₂O]_(p)CF(CF₃)—, where x is a number from 1through 10, m designates a number of perfluoroethyleneoxy units, ndesignates a number of perfluoromethyleneoxy subunits, and p designatesa number of —CF(CF₃)CF₂O— subunits; and exposing a portion of the corelayer to at least partially cure the photopolymerizable composition andform the light-transmissive patterned region.
 30. The method of claim 29wherein the core layer has a refractive index, the method furthercomprising the steps of: applying a polymerizable composition to thecore layer to form a cladding layer; and at least partially curing thecladding layer, the cladding layer having a refractive index lower thanthe refractive index of the core layer where adjacent one another.